Supplementing

Lee'A

So my little lady is 2 weeks old and hasn't gained back to her birth weight. My pediatrician is wanting me to breastfeed and supplement 2 oz of formula after I feed her for 15 minutes on the breast. So my question is how do I do this and keep my supply up? I don't want my milk supply to decrease because I have to supplement. I feel so defeated, I thought breastfeeding was going so well.