I have created an Azure App Service Environment with 4 IP Addresses but all of them are unassigned. I have a bunch of app services and I need to have one IP per app service.
I figured it out. You need to upload a SSL certificate (you can buy it in Azure portal or from any other company), import it and bind it. When you bind it you say it's an IP Based SSL. After you save you get a dedicated IP for that app service.
More on https://learn.microsoft.com/en-us/azure/app-service-web/web-sites-purchase-ssl-web-site