How to play a vanilla music disc track with a procedure

So I'm trying to use procedures to play one of the vanilla music disc tracks using procedures, and I know the play sound procedure block already does this, but I need it to play a sound from a textfield in a GUI. So basically there's a textfield in a GUI I've added, and the player will input the name of a song, e.g. wait, stal, blocks, whatever. Then they'll press a button and that song will play.

But I don't seem to be able to add a string block into the sound selector, so I can't make it play whatever they typed. I also tried using the "execute command" block with the /playsound command, but that didn't work either and I'm not sure why.