Nintex 365 Form Lookup connecting to an External BDC list on site

[fb123]

I've got a list of clients (external list) :

An I'm trying to connect to the list with a Lookup control:

However when I open the form and try, the dropdown is empty.

When I try a REST test it works as expected

....sharepoint.com/sites/Proposals/_api/web/lists/GetByTitle('Clients')/Items?$select=ID,ClientCode,ClientName

[eharris04]

You may have to use javascript to achieve this. I do not think it's possible to get that data via a rest call and populate it into a dropdown form control. It may be, but I've never seen that happen directly. 

Try using javascript if you can.