When you do so, there will be a message displayed that will say that you joined a userchannel1 (or 2, 3 or 4). Note what that number is. Let's assume it's 1. If you have joined user channels such as GLFF before, the number may be higher.
Now, in order to post to that channel, simply type /1 [message].
Say hello and let people know what kinship you are with so that we can get to know each other better.
If you have any difficulty with this process, please ask an officer for help.