Hello and Welcome. You can post on mobile. I don't use the app so not sure how to do it. However, this post of yours, posted, not sure what you used to do it, mobile or computer.
Well, first of all you have to let me know if you want to be "Richy" or if you want to be "Crazytortoise". You can't have two usernames.
Then, secondly, just go to the Forum's home page and choose the section you want to post in. When that comes up on the screen, choose "Post New Thread" from the upper right corner of your screen.