Yup, you post it in the Role-play Corner if it's an IC Thread, and in the Roleplay Lounge if it's an OOC Thread. You may post one of each if you want to, which is what almost everyone does, then you wait a couple of days until Skymin updates the Check to See if Your Roleplay has been Accepted Thread, she'll tell you if it was accepted or not, and if it wasn't, she'll tell you what it needs to be approved. Once it is approved, it'll show in the forums for everyone to see :)