How to update the lookup field of a record using Zoho CRM Client Script.

How to update the lookup field of a record using Zoho CRM Client Script.

I have two modules:
  1. Payments_Received
    1. In this module there is a lookup field to Instalments
  2. Instalments
    1. In this module there is a lookup field to Payments_Received
What I am trying to do:
  1. I want to setup a client script on the edit page, trigger is onSave, where when the editing the Instalment and selecting a Payments_Received record in lookup field, I also want to update the Payments_Received record's Instalment lookup field with this Instalment.\
  2. But I dont know what to use as the assignment when using the updateById function?
Example:
  1. var payments_received = ZDK.Apps.CRM.Leads.fetchById(payments_receivedID);
  2. payments_received.Instalments_Association = ??????;  // <<--- This is a lookupfield of the payments_received record, I want to update this field so i can update with the a link back to Instalment
  3. var response = ZDK.Apps.CRM.Leads.upadteById(payments_received, instalmentId);