Hi,
I will suggest you to review the Azure B2C Solutio for providing the access to the application this applies to external users.
Also for your onpremise app if you have ADFS you can extend it to Azure and this way you can allow access to the exernal users via Azure B2C - https://learn.microsoft.com/en-us/azure/architecture/reference-architectures/identity/adfs
Hope this helps.
JS
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