The rules read:
Advertising & Requests
Members are not allowed to create topics or posts intended only to promote or advertise their own or any website(s) without staff permission. If permission is given members are required to provide the name of the staff member who allowed the advert in their post.
As long as the advert is non-obtrusive and does not contain links to warez of any kind, members may link to their own website, forum, or venture in their signature and profile ONLY.
I would imagine that, with the permission of a moderator, you could link to your Discord in your signature or profile. I think a Discord server counts as "forum or venture." Do contact a mod to be sure.