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
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.
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