Due to change in the april 15th resolution in 2020, you no longer need to ask for release but just inform the university that you withdraw your acceptance if you get dominating offer on 16th. So if things go south and the WL go over 16th, you can accept the offer you have and later email them to withdraw. But for good will, I guess it would be always nice to send email politely.
"Applicants are not required to obtain a formal release from the program whose offer they accepted, either before or after the April 15 deadline. Once applicants have informed the program that they are withdrawing their acceptance of the offer, they then can accept any other offers."
Fingers crossed for everyone!