Thanks.
If I remember correctly, you need to activate your code directly in the League of Legends client.
Open the client, and click on the Store. In the Store, you should have a “Purchase RP” button.
In the payment options, you should have the option to activate “Prepaid Cards and Codes”. Click on the option on the left and enter your code.
You should be able to enter and redeem the Microsoft Rewards' RP code.
Keep me updated !