Board Thread:Support Requests - Getting Technical/@comment-1416490-20160510212742/@comment-1757994-20160517082347

ChatHacks has private room creation via the  slash command. (I have a fix pending review for a bug that appears to have existed forever.) It also has a feature to block and to unblock people sending you private invitations.

If you just want a button that pops up a dialog, something like would work. The above is an amalgamation of code from various sources.