Atlassian Access uses a static file at atlassian-domain-verification.html for its domain verification. This just goes at the root level of the domain you're trying to verify - example:
Details on HTTPS verification (or alternatively, TXT record verification over DNS) are available on our domain verification document.
I'm not totally sure that's what you needed though given your more recent question about SSO with Bitbucket Cloud and Azure AD.
Let me know if you still have questions - always happy to help!
Daniel | Atlassian Support
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