To redeem a Riot Games gift card in Europe, please follow these steps:

Go to the Riot Games Shop and log in with your account credentials.
Enter the redeem code in the designated field.
Click on the Submit button to claim your in-game content, including characters and skins, for your favorite Riot Games.
Alternatively, if you have a prepaid gift card, you can redeem it by following these steps:

Open the League of Legends client and log in with your account credentials.
Click on the Store button and select the Purchase RP option.
Choose the Prepaid Cards payment method and enter the pin code from the back of your card.
Click on the Submit button to complete the transaction.