How can I set the 'Related To' field of a call record using the Web API?

Hello All,

I'm working on an application that integrates with SugarCRM. One of the things I need to do is create a call record using the Web API. I'm able to create the record without a problem but what I can't seem to figure out is how to populate the "Related To" field of a call record with an existing contact. I'm able to add a related contact to the "Guests" list but no luck so far on the "Related To" field. Any input is greatly appreciated.

Thanks,

Seth