Say, a player wrote into the text box, and then clicked "confirm" button. So, get <<on "confirm button" click>> condition, and then use action "Set Value" (choose the variable you want), and set its Value to textbox.Test .
Just make sure that your textbox is actually the one that has the value you need, and it is the only one on the layout. There are ways of targeting specific textbox objects, but for now try to do it only on one.