Use that code:
SET !VAR1 EVAL("var letters = ['a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','w','x','y','z']; var string = ''; for(var i = 0; i < 10; i++){string += letters[parseInt(Math.random() * 25)]}; string")
You can add some another letters or number