Hi and welcome!
I really advise not reward training birds to poop on command. It's not healthy for them - there's a reason they poop ever 10 minutes or so. Could you imagine being told to withhold all bodily functions until someone said okay? You'd feel so sick and uncomfortable! Try thinking about it that way, and then it makes sense why we have to accommodate them.
Eventually, through voice and simple timing and placement, your bird will recognize what are "safe" zones. Avery knows she shouldn't poop on us based on our reactions (saying, "ewww", frantically saying, "no, no, no!" And placing my hand under her cloaca to stop her and immediately moving her, etc.), and if she can she will hop off and go on a towel. She also knows towels and cage = safe zones. Just be patient and respect that your bird NEEDS to go that often, and simply encourage her where it's okay to go.