I found the direct link
/new-topic?category= really useful but it has two problem when not logged:
It redirects to the main page instead of the category so if the user close to login popup he is lost (I think it redirects to the category only after the login - New Topic URL - Lost if not logged in )
It doesn’t explain why the login popup. Maybe a subtitle like “You need to login to create a new post” could be added.