WORK ESTIMATE

Creating WORK ESTIMATES for my HVAC business. Each print needs to generate a different number. My grandson , who is in Europe and not available, provided me with a javascript which he said would work. But I cannot figure out how to insert. Help!!!!

SEE JAVASCRIPT BELOW:
var f = this.getField("Tracking Number");
if (f.value=="") f.value = "7110";
else f.value = util.printf("%04d", (Number(f.value)+1));