You can start threads (also called topics in some forums) in any forum you have access to by browsing to the specific forum you want to post in. The Start Thread button should appear in the top right corner here.

If the Start Thread button does not appear, make sure you are logged in, and if you are, make sure you are a member of the campaign, as only those who are playing in the campaign are allowed to post.

Special: Some forums allow for 'guest posts', posts by players who are not members of the campaign. This is often to allow new visitors to request joining a community. Such forums are marked in the forum overview with "Guest posts welcome" behind the forum name.