I've worked with mine to only go on perches. (I have yet to get them to differentiate between a perch and the perch-like back of a chair! Smh. But I digress.) Jolly came to me largely potty trained (go, Laura!), so I'll tell you what I did with Maya and my dearly departed Bixby.
While on their tree stands, I'd watch them closely. Whenever one would go, I'd say "poop" (you can substitute whatever phrase you'd like. I'd even considered going with "bombs away" or "drop it like it's hot", but then decided to keep it simple, instead. Hahaha!) and then tell him/her that they did a good job. After enough repetition, the association was made between the word and the action, as well as the appropriateness of where said action was taking place.
At the same time, I was also timing the intervals between one poop and the next. For Bixby, it was roughly 15 minutes between one and the next. For Maya, closer to 30. So now I knew my windows. And whenever I had them on me, I kept myself aware of those windows. For Bixby, by the 13 minute mark I'd put him on his perch and wait for him to go. Then I'd say "poop", praise his action, and pick him right back up. For Maya I'd do the same, except that I'd do so at the 28 minute mark rather than 13.
Once Bixby made the association, I was able to relax a bit. Because once he felt the urge to go he would... mostly... remember to fly off to his perch, do his thing, and then fly back to me. This earned him high praise.
Maya, on the other hand, never fledged. And as such, is not very inclined toward taking flight. (Though I haven't given up on that just yet.) So I try to remain vigilant with her time and aware of her body language. I know that April/Kiwibird has trained her Amazon, Kiwi, to simply raises his "hand" when he's ready to go, but Maya, diva that she is, has refused to follow his example. Ah, well.
And Jolly, like Bixby before him, just flies off to one of the tree stands. Or his cage. Or the back of a chair. (Grumble! But at least he turns so that it hits the floor, rather than the seat! Hahaha!) And he does so with rather impressive consistency. Maybe 95%? So it's definitely doable.