Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
I can confirm that the mxRecordValue from the URL is the correct number you would enter as part of the MX and TXT records mentioned.
Things that might prevent Outlook from completing setup of your domain within Outlook premium setting could be a competing MX record with same or higher priority value. Another thing is the attempted use of a subdomain, when entering it in the domain validation prompt and in the MX record.