For the last two years I have participated in a silent auction, @Dawn150 I chose to specify an amount rather than specify nights because in my area there is a wide variation in the nightly rates due to several high demand dates.
If you provide an Air BNB gift card, is it possible for the guest to use it for any booking and not necessarily for your place? If that is your intent, then a gift card can be used according to Air BNB rules.
I provide a letter to the charity participant with a link to my listing, a specific code identifying the offer and instructions for contacting me.