By  default  a link such as  <a href="https://app.roll20.net/campaigns/details/1234567/s" rel="nofollow">https://app.roll20.net/campaigns/details/1234567/s</a> ... is   only   visible to players   already   in the game, and only  Plus and Pro  subscribers   can   make their games  visible to the public , but even then people not in the game can not post to the forums.  The best way to handle this situation is to  create a Looking for Group listing ; even if you do not enable it for viewing in the LFG Directory you can share the  link  with others and have a separate  Listing Discussion forum 
 just for those potentially interested in your game.  As a bonus, anyone
 who posts to your game's Listing Discussion forum will have a  big blue button  by their username and all a
 GM has to is click 'Invite to Game' and an invitation will be sent their way.  I hope this helps, happy gaming! :)