Set externalGroupID on Agreement creation

The Adobe Acrobat Sign Search (https://helpx.adobe.com/sign/using/adobesign-search-users-agreements.html#HowSearchWorks) discusses how to search for agreements by externalGroupID. It say the External Group ID is passed in the call to the agreement creation API. However, I don't see field on the /agreement POST to pass the ExternalGroupID.  Has anyone had success settting the externalGroupID.