Follow these steps to redeem your Kinguin Gift Card:

  1. Sign in to Kinguin and go to Redeem a Card page
  2. Enter the Kinguin Gift Card copied from the "Withdrawals" section in your Idle-Empire User Panel and click Redeem
  3. The value of the card will be converted into Euro balance
  4. Buy games on Kinguin and pay with your Euro balance

Note: You can find the official instructions here.