To use Bankin' sponsor code and get 30 days of Premium for free, the sponsored person must add the code directly from the welcome email received when creating his/her Bankin' account.
Once on this email, just click on the Add my sponsor code button to enter the code.
If you do not have access to your mailbox or don't find this email, please contact the support team at support@bankin.com and share the sponsor code with us so we can help you out.