When you create an Azure API Management service instance, Azure assigns it a subdomain of azure-api.net. you can expose your API Management endpoints using your own custom domain name, such as contoso.com.
certificate
How to create authentication certificate (Base-64 encoded X.509 .CER) using PFX certificate
If you have already created pfx certificate then there are 2 ways to generate .CER certificate. As you know pfx certificate will have both private and public key whereas authentication certificate (.CER) will have the public key only.