In order to activate your product on the Nintendo website, please follow these steps:

Activation through the website:

1. Log into your Nintendo account or create if you do not have one.
2. Go here for access to the Nintendo redemption page.
3. Paste the key into the designated area.
4. Press the “Next button”.

Activation through the console:

1. Select Nintendo eShop on the HOME menu to launch it.
2. Log into your Nintendo account on which you would like to redeem the code.
3. Select “Redeem Code” on the left side of the screen. 
4. Enter the key to finish the activation process.

Here you can check how to activate the Nintendo Switch Key: