There is no way to earn XP on the forums any more. Back in the haydays of the Red Scenarios Admins use to give XP for Rping battles. The only thing you can earn on the forums is CP's and that is by adding descriptions/Art for missing info.
*Edited for typos
Edited By: Skeltsco Ofsseto on Year 15 Day 213 20:59 ____________
Your first forum post awards you with XP and an achievement but that's it for the forums here. These forums are mostly OOC, so your character wouldn't receive XP for posting. You can register an account on our White Scenario board, linked at the top of the forums (and hosted offsite on Yuku) to participate in roleplaying scenarios for XP, though. However they are for the patient as they tend to take months or more to finish and there seems to be a bit of a backlog for XP to be rewarded, so I would only recommend that if you are more interested in the RP than in the XP.