When you create an Azure API Management service instance, Azure assigns it a azure-api.net subdomain (for example, apim-service-name.azure-api.net). You can also expose your API Management endpoints using your own custom domain name, such as contoso.com. This tutorial shows you how to map an existing custom DNS name to endpoints exposed by an API Management instance.
go to Azure portal>API Management service>Deployment + infrastructure>Custom domains>add custom domain
Configure a custom domain name for your Azure API Management instance
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