Recommendation Letter Submission
To ask such a question, just ask. Don't worry too much about &quot;polite&quot;. Just be direct. Let them know that your potential letter writer won't create an account for this and prefers email. Ask if it is possible.
Some institutions will agree and find a way. Others will not. Then you have to consider other options. Try harder to find letter writers or find institutions that have a more compatible/reasonable application process. You have little control over either the rules of the institution or the reservations of your letter writers.