In reply to Shinxtails:
You’d want to look at the top of the screen under your battle net name. Next to the three dots on top of each other you’d see your current posting toon. Click that and you’ll look down at bottom on the drop down and change your posting char from there.
To add to what Zenjy said, which characters you have available is based on which characters you have logged into recently.
Please note that Burning Crusade Classic Anniversary characters aren’t showing up the way they are supposed to. If you don’t see any characters try logging into Modern World of Warcraft, on a character that is level 10+. Once you log out of the game that should refresh your forum profile, making that character available.
Log out/into the forums should refresh it and allow you to select the character. It can be a bit temperamental, so you may need to give it some time.