Populate a drop down list in one form with items from a subform in another form

Populate a drop down list in one form with items from a subform in another form

Hi all, I'm new to Zoho Creator and could use a little help. I have two forms. For simplicity, let's distill them down to what's described below. The field I'm trying to populate (with Deluge code) is highlighted in red. 

Form #1: Orders
- Client Name selector: This is a regular lookup field that is populated with all the Client Names from the Client form below. I use this to select the Client for whom an order is to be made.
- Client Users Names selector: This is a drop down list containing the names of all the users of the Client selected in the lookup field immediately above.

Form #2: Clients
- Client Name: Simple text field.
- Client Users: Subform containing a list of all the client's users. (Each user record will have basic contact information: name, email, phone number.)

In the Orders form, when we select a particular Client Name, I'm trying to figure out the Deluge code that will populate the Client Users Names drop down list with all the users names for that selected Client. 

Example
In the Clients database, we have the following client (one of many clients):
Client Name: ABC Company
Client Users:
 - John Smith - jsmith@abc.com - 555-123-1111
 - Hedda Lettuce - hlettuce@abc.com - 555-123-2222
 - Frank Stein - fstein@abc.com - 555-123-3333

I'd like to set up an order for ABC Company so I go to the Orders form and select the Client Name "ABC Company" from the Client Name list box. When I then click on the Client Users Names drop down list box, I'd like to see the list of ABC Company's user names: John Smith, Hedda Lettuce, and Frank Stein so that I can select one of them for the order.

Thank you in advance for any help you can provide here.