[SalesForce] call conga composer from APEX

With the Callable interface that was introduced in Winter '19 you can now build a light weight interface for the methods you want to dynamically call from a class.

The example below is from the docs (tweaked to show dynamic method naming):

Example class you want to dynamically call

public class Extension implements Callable {

   // Actual method
   String concatStrings(String stringValue) {
     return stringValue + stringValue;
   }

   // Actual method
   Decimal multiplyNumbers(Decimal decimalValue) {
     return decimalValue * decimalValue;
   }

   // Dispatch actual methods
   public Object call(String action, Map<String, Object> args) {
     switch on action {
       when 'concatStrings' {
         return this.concatStrings((String)args.get('stringValue'));
       }
       when 'multiplyNumbers' {
         return this.multiplyNumbers((Decimal)args.get('decimalValue'));
       }
       when else {
        throw new ExtensionMalformedCallException('Method not implemented');
       }
     }
   }

   public class ExtensionMalformedCallException extends Exception {}
}

Unit test demonstrating the dynamic calling

@IsTest
private with sharing class ExtensionCaller {

   @IsTest
   private static void givenConfiguredExtensionWhenCalledThenValidResult() {

      // Given
      String className = 'Extension'; // Variable to demonstrate setting class name
      String methodName = 'multiplyNumbers'; // Variable to demonstrate setting method name
      Decimal decimalTestValue = 10;

      // When
      Callable extension = (Callable) Type.forName(className).newInstance();
      Decimal result = (Decimal) extension.call(methodName, new Map<String, Object> { 'decimalValue' => decimalTestValue });

      // Then
      System.assertEquals(100, result);
   }
}

Here is a link to what Conga Support recommends. Recomended approach 2 is what your asking about regarding the Partner_Server_URL and in my view better than approach 1 they describe.

So what you tried in your question should actually work, {!$Action.Partner_Server_URL_210} returns a Visualforce domain in a VF page contect, which does look odd, however you can actually use these domains for API calls. I've come across this when using URL.getSalesforceBaseUrl to call the Metadata API.

From Apex another approach is to use the (relatively new) method on the URL.

URL.getSalesforceBaseUrl().toExternalForm();

URL.getSalesforceBaseUrl().getHost();

etc..

If the variant of the Salesforce base URL returned by the above in Visualforce contexts does not work for you. You can try this formula. However be warned it assumes that Salesforce will continue to utilise the c.instance.visual.force.com format. But for now it works!

{!SUBSTITUTE(SUBSTITUTE(LEFT($Api.Partner_Server_URL_210, FIND( '/services', $Api.Partner_Server_URL_260)), 'visual.force', 'salesforce'), 'c.', '')}

I would still be inclined to ask Conga Support why there advice is not working for you though. ;-)