From the mutt manual:

Once you have tagged the desired messages, you can use the tag-prefix operator, which is the ; (semicolon) key by default. When the tag-prefix operator is used, the next operation will be applied to all tagged messages if that operation can be used in that manner.

But it seems that I can only execute commands that are already bound to a specific keyboard shortcut. For example I can use ;d to delete all selected messages.

What if I want to apply an "unbound" command (such as purge-message)?

I have also tried using something based on :exec tag-prefix or :push tag-prefix without success.