Check the URL column after adding
The list shows the URL each identifier resolved to. If a link goes somewhere unexpected, the identifier is almost always a full URL that was pasted where a handle belonged.
Social links put your group's other accounts in the footer. You give the platform and the identifier; the full URL is assembled for you.
Social in the group settings lists the links already configured and offers a small form to add another. Twenty-one platforms are supported, from YouTube and Instagram to Mastodon, Bilibili and Patreon. Each link is placed in one of the four footer columns, so the footer stays yours to arrange.