You have to be careful with Conures. You don't really want them holding their poop too long waiting for some command.
In general, they will need to poop every 15 minutes or so. So when I'm handling the bird, or have her on my shoulder, I stop what I'm doing every 10 minutes or so and put her on the perch where she can poop safely.
As soon as she eliminates, I reward her by putting her back on my shoulder, giving lots of praise, a treat, etc.
If you do this consistently, they will get the message. The faster they poop when you put them on the perch, the faster they get rewarded. I wouldn't really use a command. Eventually you will pick up on subtle body language indicating they need to get to the perch. Mine realize they need to be on the perch to get rewarded.
Its not a perfect system and I do get pooped on, but its better than nothing. These little birds cannot hold it like the big birds can.
CD