Breastfeeding v/s formula

Hi all lovel people here

So I'm first time mom and my 4 months old who doesn't take any of the formula when she born she was taking formula but after that she only takes breast milk and if you add some creal powder inside she will drink but if you add formula in breastmilk she won't take it..

Her weight(12lb now) is gaining slowly so just that's the reason why i want to give her formula once or twice in a day🤷 is that ok as she is completely healthy and active baby so I'm bit confuse

Any Suggestions please

Thankyou