You can add a voucher code up to 7 days after registration. Go to Settings > Voucher.
You must verify your account before a voucher code can be added. Read more about that here: I want to verify my account, how do I do this?
If you enter a valid voucher code, the credit will be added to your Vault immediately.
Note: in order to add a voucher code, make sure the app is updated to the latest version.