You can get a Textbox object in like "character creation" page or whatever where they enter their chosen name. Then you could also have a normal Text object pinned (Pin behaviour) or using something else to position the Text object always above the player (ergo use the position of the player to position the Text object, like Text.Y = Player.Y -100 .Then you should set Text to whatever was entered in Textbox and then destroy Textbox, or put invisible etc.
Maybe this is even better: just make a global variable that's being set by whatever the user enters in the Textbox object, then destroy the Textbox on like "start game" or "enter name" button or something, then have the Text object (above the player) be set to the global variable. That way you can also use the global variable to be referenced on other places.
Hope this helps .