Since you purchased ESEA Premium through Steam you will need to use Invoice ID from Steam in order to activate your Premium purchase. When you completed the purchase of Premium, Steam gave you an invoice ID on the confirmation page and in the receipt emailed to you.
You may use your Invoice ID here to redeem your Premium access. If you lost the Invoice ID or deleted the email from Steam. It is also available on the Steam website under recent purchases.
You will only be able to activate a Steam purchase with the same Steam account that is connected to your ESEA account. It is not possible for us to swap Steam purchases between Steam accounts and if you purchased it by mistake on the wrong Steam account, you will need to refund the pruchase through Steam.