To configure a custom domain for an Azure App Service, what must be done first?

To configure a custom domain for an Azure App Service, the initial step involves ensuring that the application settings are properly configured. This is crucial because the application settings include the necessary configurations that define how your Azure App Service operates. Configuring the custom domain often requires you to set the default domain name in the application settings, and these changes help Azure recognize the incoming domain requests and respond accordingly.

Before any custom domain mappings are created, you need to make sure that they are reflected in the application settings, facilitating a seamless communication between the Azure platform and the domain you want to use. This laid-out configuration is essential for establishing a functional connection between your Azure service and the custom domain, so it can handle web traffic correctly.

Other options, such as scaling the App Service plan, while they might enhance performance or capacity, are not prerequisites for setting up a custom domain. Similarly, adding a continuous WebJob, which is a background task feature within Azure App Services, does not relate to the domain configuration requirements. Scaling out the App Service plan, which usually involves increasing the number of instances to handle web traffic, does not affect domain settings either. Hence, focusing on configuring the application settings first is the logical and necessary step.