Linking microsoft accounts is no longer supported.
https://support.microsoft.com/en-us/help/4028404/microsoft-account-linking-accounts
what you can do is create an alias and logon with that.
https://support.microsoft.com/en-us/office/add-or-remove-an-email-alias-in-outlook-com-459b1989-356d-40fa-a689-8f285b13f1f2
--please don't forget to Accept as answer if the reply is helpful--
Regards, Dave Patrick ....
Microsoft Certified Professional
Microsoft MVP [Windows Server] Datacenter Management
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