i would like to add a button on my checkout page for people to apply the amount of their gift balance to pay for their order total, i dont want this to be automatic just an option but i really dont know where to start, i guess the button should check their balance and insert into the box, they would still use the continue checkout button as normal, all i literally want the button to do is apply an amount that is available in that text box, nothing else.

could someone please help me with this?

thanks in advance :)