When configuring a custom domain for an Azure Web App (App Service) such as webapp1, the first prerequisite is to create and configure a corresponding DNS record that maps your custom domain name (www.contoso.com) to the Azure App Service's default domain (webapp1.azurewebsites.net).
According to Microsoft Azure App Service documentation:
"Before you can assign a custom domain name to your web app, you must first create a CNAME record in your DNS provider that maps your custom domain (for example, www.contoso.com) to your app's default domain name (webappname.azurewebsites.net)." This DNS record ensures that requests to the custom domain are routed correctly to Azure's front-end load balancer hosting your web app. Only after the DNS record has propagated can you validate and bind the domain to the web app in the Azure portal.
Uploading an SSL certificate is only required for HTTPS bindings after the domain is added. Stopping the app or adding connection strings is unrelated to domain configuration.
Therefore, the first step is to create a DNS record at your domain registrar that points to your web app.