In the Azure portal under your Azure Active Directory tenant, select App Registrations
Click "New registration" Give that application a meaningful name Press "Register"
In your new application select "Certificates & secrets" and then "New client secret"
Give the secret a meaningful description and set the expiry date
Copy that secret and use that for the ClientSecret item in the configuration, in the overview the Application (client) ID is the ClientId and the Directory (tenant) ID is the TenantId
Go to your Front Door resource in the Azure Portal
Select "Access Control (IAM)", then "Add a role assignment"
Select the role "CDN Profile Contributor" then "Next"
Click "+ Select members", search for your new App Registration and select it
Click "Review + assign"