As i understand you are unable to verify one of the domains that you own in your tenant, as this domain is still verified in your old customer tenant.
Also, you are getting an error while trying to add app ID URI to application in Entra portal.
IdentifierUris is the user-defined URI(s) that uniquely identify a web app within its Microsoft Entra tenant or verified customer owned domain.
For a single-tenant application, the App ID URI need only be unique within that tenant. For a multi-tenant application, it must be globally unique so Microsoft Entra ID can find the app across all tenants.
For example, if the name of your tenant is contoso.onmicrosoft.com, then https://contoso.onmicrosoft.com/myapp
is a valid App ID URI. If your tenant has a verified domain of contoso.com, then a valid App ID URI would also be https://contoso.com/myapp
.
In your case you can use your default domain name for your tenant for now.
About the domain that is still verified in your old customer tenant, you will have to contact old customer to get it removed from there tenant. If you are unable to reach them then you can try below steps to reach our data protection team.
You can look into below article to get support numbers depending on your country.
Once you create a ticket with support team you will have to work with our data protection team. You will have to first prove your identity against your tenant for security purpose. Post that this team will help you with domain removal or they will help you with other ways to contact your old tenant owner.
Let us know if you have any further questions.
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