How can I combine functions with textbuttons?

So if I was to create a textbutton, how would I make it so when they are finished typing in the text of a player, and press enter it checks to see if the player's in the game ( a for loop ), and if it is it adds a checkmark to the right of the player ( imagelabel + fadein with transparency ). So I know how to make all the stuff pop up, when the scan is finished and it's found the player. I just don't know how to make the scan start when that player presses enter on a textbutton and has typed a correct username in.