Word puts the page number field in a FRAME. I see a few options to manipulating, but not particularly one-step.

1. Go to the footer and INSERT FIELD, NUMBERING, PAGE. That leaves out the frame, so it is easier to add/format your C-2-

2. You can insert the page number as you normally do, go to the footer, and carefully insert your cursor inside the frame, not highlighting the page number and add in your prefixes inside the frame

3. Insert the page number as normal, go to the footer, right-click on the frame surrounding the number to Format the frame, and then remove frame. Again, you can manually add in your prefixes