How do I make the game display the username of someone who pressed a button?

[That One Bird]

I tried {CLIENT_NAME}, #username and etc.

 

[Zeus]

You might be able to do stuff with !activator, but im not sure how you get the client name from that.

 

[Da Spud Lord]

There is no way to obtain a player's username using map logic exclusively. This is the sort of thing that would only be possible with a sourcemod plugin, sadly.