Event Creation via API - Related to Potential

Event Creation via API - Related to Potential

I'm able to create an Event just fine in using the Zoho API from a java program. But I would like to populate the Related To fields with a Potential rather than a Contact or Lead and I am not able to.  Is this feature only available from the Web interface? 
Could someone provide the required elements I would need to accomplish this (xml or json)? 

The documentation says I can specify the 'Account' but the description says: " Select any other record related to the event" 

thanks,