But the way I did it is I copied and pasted the entire BBC Code on the first box below the form you fill out. Highlight the code and then right-click 'copy'.
Then, at the bottom of this page while reading this
thread, look where it says forum jump. Open that window and do a forum jump to 'user control panel' which is near the top of the long list. Once there, you see on the left where to click to update your signature, and that is where I right'click 'paste' the BBC code for your banner from the banner page.
There are probably more automated ways to do it, but I did it manually like this and it worked.