How to use Limited Capsule Coupon ?

[76561198418606418]

I got 3 of them but I have no idea how to use them. I press the big button "Use" in the inventory and nothing happen

[D34Done]

You need to get a Limited Capsule box in order to use them.
Pro tip: Use the search function and look for the low number boxes that still have A and B tier items to use the coupons.
You will need 2300 first to acquire the searched box though.