Throw Reject Misses are when you make the opponent try to break a throw at the wrong time, thus making it so they can't avoid it. The easiest method to do this is using two controllers and setting "Grab" (B+C) to left trigger on your main, right trigger on your secondary. The characters I had used were Ragna and Lambda however I don't think this will make a difference.
The order you'll want to do things is Square with your main, then quickly press R2 on your secondary (grab) and press L2 on your main in order to do the actual grab. The timing is a bit tricky, however you'll get in to the motion of it after successfully pulling it off once or twice. You'll know you've done this when it says "Throw Reject Miss" under the combo counter and damage dealt.