1. Log into your Prembox account: https://prembox.com/
( If you don't have an account, please register a new account: https://prembox.com/register.html )
2. Go to "My account" : https://prembox.com/account
3. Click on "Add traffic" on the left column of the website.
Paste your Premium key / Premium code in "Received code:" , and click on "Recharge" button to proceed.