Thank you for your post.
At this time, FileMaker Go uses the built-in iOS fonts. For a complete listing of these fonts, see:
Unfortunately, Code 39 is not one of these fonts. That may be why the makers of CNS Barcode create a graphic.
Also interesting would be to compare the IOS fonts that Go uses with:
The fonts that the themes use to see how they match.
TT fonts available on Windows computers as there may not be matchiing fonts available for install in Pro.
TT fonts available on Macs and assume that these would all be available and useable in Pro.
Strange as Code 39 is widely used by many people.