It must be a panel with a blue holographic screen showing the required droid. The small circle ones don't count.
This is the earliest you can do it:
1. On Chapter 7, Leia will eventually ask you to activate a panel with BB-8. This will directly after you need 4 people to stand on the floor scanners. Once there, simply use C-3PO on the panel.
2. Play through "Poe to the rescue" until you are in the corridors. When you reach a panel, simply use the other droid on it.
3. Land on D'Qar and enter one of the hangars and use the wrong droid on any panel. (after story is complete)