You can look for emails from Cloudflare about your domain that you may have had in the past.
Otherwise you’ll need to add your domain to a new Cloudflare account, change the nameservers at your registrar to make the domain active in that new account and set up your DNS again.
I show emails as being delivered to the address you are using here and the address in your topic.
I also show the domain you shared as pending a plan selection in the account you shared in your topic.
I show the nameservers for the domain you shared as
darl.ns.cloudflare.com
olga.ns.cloudflare.com
That pair of nameservers are not associated with the account you are using here nor the account you shared, that indicates there are more than 2 accounts involved with that domain.
As mentioned by @sjr, he only way you’ll be able to regain access is by having access to the email address of the account.
This link may help, but they only work when logged out
We have tried forgetting the email, but we did not receive any emails from cloudflare.
If I transfer starcharger.com.tw by transferring the domain, will all the records be completely transferred over? What we are worried about now is that our original services will be interrupted after the transfer.
Is there a way to directly modify the registered email address of the starcharger.com.tw domain through the backend?
If adding the domain to a new account you can import records that are known through a DNS search of typically used subdomains. Any subdomains not in the Cloudflare test list will not be found and any proxied records will be brought over as Cloudflare proxy IPs and not work. Cloudflare will not internally transfer the DNS records over as that would be a security risk. You will need to ensure you have all the DNS records a imported and entered correctly yourself.
No, that would be a security risk. Proof of domain ownership is by being able to set the nameservers for the domain to point at those for a Cloudflare account you can log in to.