Hey guys, i wish to ask regarding shopee purchase discount. (how does it apply?)

Normally when we purchase in shopee, to get discount etc, we have to key in voucher code and the amount deduction will automatically reflected.
But what about this citibank promo?

It says 15% discount, but how will it be reflected in shopee?
Does anyone here have experience using citi credit card for shopee purchase with discounts?

Thank you