I do a lot of equestrian photography and sometimes run seminars for folks wanting to learn to shoot various events.
I believe your D750 is a full frame (FX) camera.... On that you'll find the 70-200 useful for a lot, but a bit short for some arenas. It depends upon how close you can get to the action, but at times a shorter 24-70 can be used, too.
Most of the time I use a pair of Canon 7D, which are crop sensor cameras ("DX" in Nikon terminology)... 70-200 on one and 300mm on the other much of the time (equivalent to 112-320mm and 420mm on your camera). I usually have 24-70 and an ultrawide 12-24 or 10-22mm in my bag, too, for occasional use.
Do your best to "fill the frame". One of the most common mistakes I see are far too distant subjects and/or too short lenses... requiring way too much cropping to make a usable photo.
Like Tommi, I don't use Nikon and only have passing knowledge of your D750. So I don't know how loud your D750 is, but full frame cameras in general are louder than croppers and I do have to be careful around horses with my full frame Canon 5DII, because of the noise of the shutter action. The sound of the mirror slap of that camera has sent a few horses "vertical". So now when I'm using the 5DII (mostly for portraits) fairly close, I'll make a point of "testing" the horse by talking with them and tripping the shutter a few times, when the camera isn't pointed at them. Most horses ignore it. But some react.
Tommi brings up some good points... It can be difficult to keep fast moving subjects in focus. Several tricks that might help...
Stop your lens down a little... f4 instead of f2.8... f5.6 if possible. This increases depth of field, which gives you a little more latitude in focus accuracy. From many angles and working distances shooting equestrian events, more DoF is needed anyway, to keep both the horse's face and the rider's face in focus.
I typically use a continuous focus mode and a single AF point most of the time. (Unlike Tommi, I do not use 7D's Spot Focus mode.... which is single point using a smaller than usual AF point for high precision, but is slower than standard single point.) I also use Back Button Focusing. I get around 95% to 98% of my shots acceptably in-focus.
Another method is to use One-Shot mode and pre-focus on a jump, then wait for the subjects to arrive and fire away when they do. I do this rarely now with modern, faster cameras. But with manual focus and some older AF cameras, it was sometimes a necessary technique.
The vast majority of that show's schedule is Hunter/Jumper... so there will surely be plenty of opportunities for shots over the fences. The route around the course can change from class to class, but in each class the contestants will all ride the same pattern. So if you watch the first horse and rider, all the others in the class should follow the same path (assuming no one goes off course).
You simply can't photograph every jump.... so find a vantage point, set up and shoot the jumps closest to you, where you have a clear view and a good angle. Experiment with shots head-on, at a quarter angle, from the side and even quarter angle going away from you. I like to mix up really tight shots of horse and rider and broader shots showing the whole jump, as well as the horse and rider fully.
Also don't forget that your camera can take shots in a vertical/portrait orientation. For some angles it's better... while a horizontal/landscape orientation is necessary for other shots. I have seen photographers who don't seem to be aware of their ability to re-orient their cameras and shoot absolutely everything landscape. Gets kinda boring and doesn't make best use of the whole image area a lot of the time.
With jumping you'll need shutter speed of 1/500 or better to freeze action (depends a little on distance, direction of movement). But it's also possible to use slower shutter speeds to get interesting shots... "Dragging the shutter" while panning can heavily blur down backgrounds... however don't expect every shot to come out. When doing this, I throw away a lot more than I keep!
Best time of day to shoot is early or late, like anything else. High sun midday cause heavy shadows. When I can, I use fill flash midday. But flash isn't always permitted and it slows down shooting (waiting for the flash to recycle). Also, with telephotos it's often necessary to use a flash extender.
Hope for an overcast day... Those can be the best, overall, for photos. I also look for shaded areas of the arena and work those, when possible.
Timing is important...
While I like all four of the above for various reasons, most riders would likely choose the 2nd or 3rd shots as the best.
The best moment can be different depending upon your angle...
Of the above, the first is obviously too early... the second shot would be good from the front or quarter angle, but in a side shot like this the horse's forelegs are partially hidden behind the near gate post. The third and fourth shots are the best, here.... the fourth would not look as good from the front or quarter view.
In the above shots, the first captures the horse well, but the rider's face is hidden behind the horse's neck and mane... no good. In the second shot the rider's face is still partially obscured, but I really like it because it shows how closely the horse and rider work together as a team. The third shot shows good technique on the part of the rider, who's looking ahead to their next jump while letting the horse handle the current one.
Don't be too fixated on the action... also check what's going on outside the arena occasionally. There are many photo ops at equestrian events!
All the above are "proofs".... very quickly batch produced to be posted online in a timely manner. In some cases I'd clean up the backgrounds and/or do other retouching, for a truly finished image.
A couple other things.
Get as close to the action as you can, and on the same level as the horse and rider. Generally speaking, shots from above, such as a bleacher or grandstand, make the jump look less dramatic than one from the same level as the arena.
For that matter, low angle shots, especially shot from close, can look really dramatic. However I only do those with horses and riders I know well, because horses can react badly to a person crouching "like a predator".
Also, if able to shoot from the fence at the edge of the arena, be careful to keep your feet, hands, camera and lens behind the fence line. Horses can freak out at "something strange" protruding from over or under the fence. Plus it's usually best to stand in clear view, so the horse can quickly and easily identify you as nothing more than another person and no threat to them.
If you have access to and are in an area with standing horses and riders either observing or waiting their turn, be careful walking behind them or startling the horses in any way. That's 3/4 ton of muscle with an inherent "flight reflex" and sharp, hard hooves that make for very effective defensive weapons!
Horses think camera bags look a lot like feed/treat bags, too. A few even think cameras and lenses are tasty (my 24-70 has been chewed on a few times!)
Be safe and have fun!