When posting within a settlement simply choose your location (ie. Muddy Rudder Bar, Weatherly Hotel, etc..) and post with that in the title. If there is already a topic for that area join it instead of starting a new one. If a topic goes too long without being updated it will be closed so a new one may be opened (although not all topics happen simultaneously imagine this being like a new day.)
If you are not in a specific location in a settlement the title can be whatever you want.