Setup URL
After you create your subscription, you need to setup your portals before you can provision them. That means that you need to specify, for each of portal instance you want to enable, a domain prefix, Dynamics/Dataverse connection details and email settings.
Setting up basic URL
Portal on Externable can use custom domain and most liekly you want to customize the address that your portal is available at. However as Externable portals are hosted in Azure, you need to define a basic URL sitting on top of azurewebsite.net domain. Once your portal is provisioned you can attach your custom domain.
NOTE:azurewebsites.net is a global Microsoft root domain, so you need ensure your selected subdomain is unique. You can do so by clicking Check availibility
Don't click save yet as you need to populate your Dynamics/Dataverse settings too. See in the next section.