The critical factor here will be how humid will it be? If it’s humid, sweat won’t evaporate, and your horse won’t be able to cool themselves appropriately.
I think this chart is probably a good rule of thumb. 100 degrees with low humidity is probably OK, but if it’s high humidity it may be best to withdraw. I would also think that how acclimated your horse is to hot weather is a factor as well. If it’s usually 80 degree highs, and this is an out of the ordinary event, then I’d be extra cautious as it’s going to feel EXTRA hot for your horse, but if your horses are used to 100 degree days, then it might not be such a big deal.
Other things to consider are if you have easy access to hoses with cold water to cool your horse off, if there will be shade available to hide under while you’re waiting, and the length of time you’ll actually be doing strenuous exercise. Low level dressage and jumping isn’t going to be as taxing as 3* XC for example (both in the warmups required and the actual competition). At the very least though, I’d definitely brush up on recognizing symptoms of heat stress in horses (and people!) though.