Cant add a value on a custom fields with the datatype of lookup

Cant add a value on a custom fields with the datatype of lookup

I am trying to assign a value on a custom field with lookup datatype but upon checking the newly created Deals, the custom field is empty. I am using PHP SDK and here is my code in adding a field value on my custom field:

$zohoCustomer = new $recordClass();
            $zohoCustomer->addFieldValue(Accounts::id(), $customer->ref_id);
            $record1->addKeyValue("End_Customer", $zohoCustomer);