Get PrincipalId when deploying ARM Template

No, this is not possible as of today (26.06.2017). This might change in the future. You can always create feedback item for that and track it.

https://feedback.azure.com/forums/34192--general-feedback

Function reference:
https://docs.microsoft.com/en-us/azure/azure-resource-manager/resource-group-template-functions-deployment

This is now possible using reference().

"[reference(resourceId('Microsoft.Web/sites', variables('appService')), '2016-08-01', 'Full').identity.principalId]"

Note the use of 'full' parameter in the reference() function.

Relevant docs: https://docs.microsoft.com/en-us/azure/azure-resource-manager/templates/template-functions-resource?tabs=json#reference

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