For a drop-down editor, you’ll need to implement a custom text editor. Please read:
GoJS HTML Interaction -- Northwoods Software
Here’s an example:
Text Editing Examples
That makes use of this extension:
https://gojs.net/latest/extensions/TextEditorSelectBox.js
Which binds the TextBlock.choices property.
I don’t know if you want to make that the default text editor, as that sample does.
By coincidence I just posted an example of how to implement your own tabbing: Simulating hover/focus on element - GoJS - Northwoods Software (nwoods.com) Of course you need to figure out what to do when the user hits the Tab key.