Assuming associatemultiuserlookup is required, the following result was found.
and here's the code to associate 1 user in the multi-user lookup: m_AssociateMultiUserLookup = {"data":[{"id":p_QuoteID, "Requires_Approval_Users":[{"Requires_Approval_Users":{"id":m_ApprovalUser.get("id")}}]}]}; r_AssociateMultiUserLookup = invokeurl [...