To use your Perks voucher you can either enter the voucher code at the bag page in the box that says “ADD PROMO CODE / PERKS VOUCHER”

or

In the “Promo Code / Perks Voucher” section located above the payment information during the checkout process. 

You can also add a voucher to your bag via the My Account section of the website here: https://cottonon.com/AU/account/