If, for example: @Trap does not work, do it with quotation marks before & after the username.
Example:
@ "Trap"
(Do not leave a space after the @ symbol. That is there just so my explanation will work.)
Usually quotations are only needed for 2 word usernames with a space in between. If your friend / member you're trying to tag has special characters in their username, you'll need to go to your message box & click the big Bell shaped icon to the right of the "Text Size" dropdown menu to tag them in a post.