Hi
You'll need to install powerbi-visuals-utils-formattingutils, and manage in your code to pull the format string from your field and apply it.
If you want specifics, I have a repo with a tutorial very similar to the circle card which covers formatting a measure, if you want to have a look. I did this in a presentation earlier this year, and also contains a PDF copy of the presentation with steps and provides details of each commit, which will help you see changes between steps. Hopefully you can find this useful.
Regards,
Daniel