I'm attempting to enroll at a different university, and they are requiring me to have a letter of withdrawal or something similar. How do I officially withdrawal and obtain this letter?

Here is the exact wording I got from this other institution

"I contacted our Admissions Office and they informed me that you will need to submit a withdrawal letter from your previous institution first indicating withdrawal in order to proceed with admission"

I've found what looks like a withdrawl form for the graduate school, but have no official process, forms, or policy concerning undergraduates. Note: I don't mean withdrawal from any classes, I mean withdrawal from the university and....apparently there is a distinction.