@Mir Mustaq Hussain
Thank you for posting this in Microsoft Q&A.
As I understand you want to assign Global admin role to one of your users in Azure AD. But your current global admin account is associated with Office 365.
Office 365 is just an application which is again part of Azure AD. This means even if you try to login to Office 365, the authentication for user happens in Azure AD itself.
Office 365 is just an application with its own user admin portal. Whatever users and user roles you have in Azure AD, it would be same in office 365 as well.
As per your question you have an account who has global admin in office 365. As per my explanation, you can login with same global admin credentials to Azure AD portal as well.
If you are the only global admin on the account and are blocked entirely, you can reach out to the Azure Data Protection team to restore access. 866-807-5850
Also, for the future, you can create an emergency access account (break glass) in Azure AD. This account will help prevent being accidentally locked out of your Azure Active Directory (Azure AD) organization because you can't sign in for any reason.
https://docs.microsoft.com/en-us/azure/active-directory/roles/security-emergency-access
Some other support option include;
AzureSupport
https://support.microsoft.com/en-us/topic/global-customer-service-phone-numbers-c0389ade-5640-e588-8b0e-28de8afeb3f2
azcommunity@microsoft.com
or creating a ticket through a different account: https://learn.microsoft.com/en-us/microsoft-365/admin/get-help-support?view=o365-worldwide#phone-support
Let me know if you have any further questions.
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