Using iCloud email domain in Mail

I purchased a domain via my iCloud+ subscription and created an email account on cloudflare. Unfortunately, I forgot what email I provided to sign in to cloudflare. All I have is the domain name and my email account. If I am able to retrieve the information, how can I use the email account with Apple Mail? When attempting to set up an account there, I am asked for a password - but I don't have a password, only the email address. Is there any way this can be remedied? Should I open a support ticket? Thank you.