extrakangaroo22:
Is this part of a waitlist or the normal application process? If the former, there should be instructions provided in the waitlist email/letter. If the latter, you need to contact the Admissions Office directly to see what their current process is for submitting additional materials.
As for the contents of the letter, again, it depends on your current status (waitlist vs. future applicant). Who is also submitting this letter? Is it you? Is it someone on your behalf?
Please contextualize the ask so we can properly advise you.