I need to register a custom domain but there is an error that says the domain has been previously configured on an existing Azure AD or Office 365. I no longer have access neither I can reset the tenant password because the restore email address is in a domain that i dont own. how can force remove the domain from the other tenant if I am the owner of the domain?
Contact support, you will have to pass some verifications, but they will be able to help you in this scenario. In addition, check for the so-called "viral signup" issue, where regular users might have "registered" this domain with some O365 service. If this is the case, you as the admin can perform "domain takeover": https://docs.microsoft.com/en-us/azure/active-directory/domains-admin-takeover