Hi @Bhavya
Thank you for posting this in Microsoft Q&A.
I understand that you are unable to Find "Add Partner One ID to Verify Publisher" Option in Azure Portal.
To add an MPN ID for publisher verification, the domain must be publisher domain or DNS-verified on the tenant and match the primary contact domain of your MPN account.
Could you please confirm whether you are using an email-associated domain or a custom domain? It would also be helpful if you could share a screenshot of the branding and properties section of the application page for a better understanding of the issue.
Thanks,
Navya