"proton.me" and "pm.me" (current) is short and good, but few services--such as PlayStation account--does not accept both domain. However, when I test "protonmail.com" (legacy) as a domain, it got accepted, but it cannot retrieve mail simply because I am new to Proton Mail, so I don't own the domain.
Anyway, about the PlayStation account, I had to use my Google Mail (gmail.com).

Please provide the option to use the legacy domain for new users. It will be useful for anyone who had similar issue as me. Sorry and thank you.