Using Text Inside Textfield To Rename Mobs

Hello, I have been struggling to do the following. So I made a GUI which opens on certain conditions and the player is to type text into a textbox when the GUI shows up, this being the name of the pet that they are to travel with. My problem is when the pet is spawned I cant seem to use the "get text inside of textfield" block to name the pet with the player's name in brackets showing who it belongs to like the name tag feature in the vanilla game. Any help would be greatly appreciated.