First of all, you have to make sure that you have met the conditions for receiving the Black Diamond 7-day Experience Voucher.
Then, enter the column of looking for activities in official website, click "More" to find the activities of "accumulating gifts online", and click to collect them.
Official website address
Generally speaking, it can be used directly. If you really can't use it, log in to the game again.
Remarks: Only one character under an account can receive experience value.