Thanks JohnKorchok,
I'm not trying to turn on field codes for one type of field, I'm trying to only display a particular type of field.
In this case, I only want to show TC fields. (Table of Contents entries)
If I used ThisDocument.ActiveWindow.View.ShowFieldCodes, this was also showing the code in REF fields as well which is not what I wanted.
I found that using the following achieves what I'm trying to do (for someone else who might find this post):
ThisDocument.ActiveWindow.View.ShowHiddenText = True
Technically, this might turn on other things as well, but in my case, it's only showing TC fields. I imagine if you had other fields which are classified as "hiddenText" they would also show, in my case, it's only the TC fields.