Immediately (or very soon) after purchase you will have received an email entitled "Your Industrial Scripts Ltd. Receipt" followed by a number.
The email itself looks like this:
The number you need to enter in the form is the one beginning with the hash.
Just add this to your form when checking in!
PS. don't think you were sent the receipt? This is pretty unlikely...it was sent by one of the world's largest payment processors. Please check your junk mail, your promotions tab, and every other folder first.