Cannot Change the Display Type Property of New Extension Field

I created an extension object to extend a standard object CustomerRequirement like so;

import AP.Common.GDT;
import AP.SupplyChainControl.Global;
[Extension] businessobject AP.SupplyChainControl.Global:CustomerRequirement {
        // You must activate this business object before you can access the extension fields
        // or messages in script files, forms, and screens.
[Label ("Ship-from Location")] element ShipFromLocation : String;
[Label ("Reason for Transfer")] element TransferReason : String; 
}

On the UI Designer, I'm unable to change one of the fields to a dropdownlist as required. I tried to change it from the display type property but it just returns to TextEdit. How can I change it to a dropdownlist please?

I am using version 1908.