An enum OpenAPI field can't be mapped to a dynamic value in the UI. There's no "Fx" button.
It can't be mapped in JSON mode either, due to OpenAPI: Nested Object field incorrectly serialized to string
I can't seem to edit the OP, but I can now work around this by setting the entire JSON object.
Just want to acknowledge here that this feedback has been passed along to our dev team and that we'll report back here when it has been fixed 
