If discourse is working correctly, you navigate to the desired forum category from here (Scouting Forums), click on the one you want (say Scoutbook Bugs - Scouting Forums), then select the New Topic button (or “+” symbol on mobile, I think).
Sometimes (for reasons I don’t understand at least), Discourse doesn’t serve the blank new post (or reply) window, and you have to click into a different thread to get it to show up. It’s a known issue that seems to be some sort of interaction between discourse and the BSA implementation of the forums, but nobody seems to have tracked it down yet despite it having been looked into.