How to assign users to Azure AD Application Proxy published sites via Powershell?

My organization has several web applications published via Azure AD Application Proxy. 

These were published via the Azure portal by going to Azure AD > Application Proxy > Configure an App. 

I have the need to make bulk user assignment changes to these apps. 

The Documentation: https://docs.microsoft.com/en-us/azure/active-directory/manage-apps/assign-user-or-group-access-portal#assign-a-user-to-an-app---powershell

References needing to use Get-AzureADServicePrincipal to use a property in New-AzureADUserAppRoleAssignment

However, none of these applications appear when i run Get-AzureADServicePrincipal

They do appear when i run Get-AzureADApplication

But if i Set -ResourceID in New-AzureADUserAppRoleAssignment to the AppID from Get-AzureADApplication, it says it doesn't exist. 

How can I assign users to Azure AD Application Proxy published sites via Powershell?

Thank You.