Should be the same as a normal one, select FNB in your list, use the credit card account number (on the FNB app it's starred out, you'll need to download a statement, the full number is on there, it's not the same as the number on the credit card).
Haven't done it myself though, so rather test with R5 or something first. Note if premiere account and up, you will pay a fee for EFT from credit card, check account it's bundled in (if you have the bundle).