In your email, clearly state that you want to delete your account
Include any relevant details about your account to help them identify it
Currently, there is no self-service option to delete your account - this requires assistance from the support team. According to official AnkiHub information, they plan to implement a user-controlled account deletion feature in the future, but for now, email is the way to go.
The support team should be able to help you with your account deletion request once you reach out to them.