How to use insert/update Records API to enter data in lookup fields

How to use insert/update Records API to enter data in lookup fields

Has anyone ever tried (I assume someone must) to insert/update the value of a lookup field to Contacts/Leads using API call? 

I have added a mandatory field to Products that is a lookup field linked to Contacts called "Seller".  When I retrieve data with the APIs I get the concatenated "First Name"+"Last Name" of the contact record as the field value for Seller.  This is good, my users can see which contacts is selling the given products. 

The problem comes when I want to insert a new product (or update the seller for an existing product) using insertRecords or updateRecords.  How do I set the value of the lookup field called "Seller".  I have tried sending a string of the first and last name of the seller (as Zoho would hve produced it), and I have tried sending the CONTACTID for the Contact record that should be linked through "Seller". 

Anyone ever done something like this:  inserting/updating a lookup field value that links to Leads or Contacts?

Thanks