Creating a hidden field in Donation Form

What i'm trying to do is create a donation form that has a hidden value or a hidden field that will pass a sub_src code thru when the visitor submits the form.

What we want to do is when the visitor is going to donate we want to give them the option to not receive a premium and if they decide they just want to donate and not receive anything they would check a box and then that check box will pass the sub_src code thru.

So when we get the data sync numbers we can seperate the group that did not want to recieve the premium.

If someone has a snippet and or sample code that would be great or if there is a better way about doing this it would be greatly appreciated.

Thanks,

Matthew