Paying with Apple Gift Card and Apple Credit Card for same online purchase

I have an Apple Gift card that I want to use to purchase iPad. I also have the Apple credit card. If I add the gift card to my Apple Store account, will that balance be used first when I select to pay for online purchase of an iPad with the Apple credit card? I can't see an option that will allow me to enter gift card serial number when trying to check out for online purchase. Or should I just call Apple to place order?