Recently, I was working on a project and I had to get the custom domain using the standard API that Salesforce provides.

On fiddling with the documentation for a couple of hours I stumbled upon few very very useful methods which serve as the Swiss Army Knife as far the request URL is concerned.

Let’s start off with a few methods and finally we will look at how to get the custom domain.

1. Get the base URL.

String sfdcBaseURL = URL.getSalesforceBaseUrl().toExternalForm();
System.debug('Base URL: ' + sfdcBaseURL );

2. Get host from the base URL

System.debug('Host: ' + URL.getSalesforceBaseUrl().getHost()); 

3. Get protocol from the base URL.

System.debug('Protocol: ' + URL.getSalesforceBaseUrl().getProtocol());

4. Get the query string of the current request.

System.debug('Query: ' + URL.getCurrentRequestUrl().getQuery()); 

5. Get custom domain.

String customDomain = URL.getSalesforceBaseUrl().toExternalForm()

Hope this is helpful!