I agree, go with a smaller ball first.
I think we taught Uni basketball with the drop it command. She already knew how to fetch, so it was just a matter of her dropping on a mark. She likes to chew her balls and squeak them on people so it wasn't much of a stretch to have her squeak it on the rim. We tried to get her to look down into the hoop by putting our hand up from underneath with a treat, so she would have to stick her head in there to get the treat. That was so she would realize the hoop around or near her face was not a scary thing.
Then we would tell her to drop it while she was squeaking the ball and at first most of the time it didn't go inside, but when it did we praised her to no end. The very first time she did it, we rejoiced to loudly we scared her, which in turn i think made her even more scared. But eventually she started getting it. Then we raised the hoop so she would have to be standing on her back legs to slam it like a proper dunk. This is where we encountered problems. She could do it, and did, but she put her hands on the rim for support and it snapped out of the backboard. That freaked her out, so now if we even try it on the higher position, she will NOT put her hands anywhere near the rim. So we have stopped doing it in the high position.
She does do it with whatever ball though we did start out with a larger ball, about 3" or so. the one in the vid is the one she started with so you can see it's quite big. It's a soft vinyl ball though, so it smashes in easily. We didn't try to use the little basketball it came with. Don't remember why.
btw, Mike, what is that contraption you have that spits the ball back out? Did you steal that from a mini golf course or something?