Don't know how Free does it, but via the new site, if you...

Don't know how Free does it, but via the new site, if you firstly go for "New Thread" from the home page, not from within the STT which doesn't allow you to do both I think. From here, you can put in XAO (gotta do it first);

Then select Short Term trading from the drop down, and it will have both in the post

If something goes awry or that's too complicated, easiest way is just make it then report the post asking for a mod to add the XAO to the thread .