Just go to the first message in the thread (on the right-hand side, there’s a slider thingie that says “Feb. 2025” at the top — click on that date and you’ll end up at the first message) and in that, notice the block that says “Shermania 2025” with three grey buttons at the bottom. Click the left-most one that says something like “Going” on it (it’s localised in Dutch for me, so it says “Gaan” in my browser, which I suppose means it says “Going” in English — anyway, you need the one on the left 
).
Then all you need to do post about your model in that same thread.