How to tag users with @mention/@tag in a line group using line bot webhook requests

I cannot find anything in line message API that shows how to tag or mention users in a group that the bot is part of.

C98a77173e4622xxxxxxxxxxxxxxxx is the group ID to which I need to send the message to and U5a4640d93cxxxxxxxxx is the user I am trying to tag.

I tried something like this:

    $data = '{
           "to": "C98a77173e4622xxxxxxxxxxxxxxxx" ,
           "messages": [
                         {
                            "type": "text",
                            "text": "@{U5a4640d93cxxxxxxxxx}"
                         }
                        ]
            }'; 

This did not work though. I tried all the possible combinations I could think of as the text content but everything just gets sent as it is and not getting converted to mentions or tags. I have seen other bots doing this and more actions like removing users from groups, adding users to groups, cancelling invitations and a lot more but none of these message types are mentioned in line Message API. Am I missing something?