There are a few possibilities to activate the PSN key. You are able to redeem your game on the website or through your console.

Website Activation:

1. First, sign into your PlayStation account. You can do so by clicking the “Sign In” button in the upper-right corner.
2. Click on the “little happy face” button to open up a drop-down menu.
3. Then, you will see the “Redeem Code” section.
4. Paste your key in the Redeem Code Window
5. Click “Continue” and your code has been activated!

Console Activation:

1. First, make sure that you're logged into your PlayStation Network account.
2. Then go to the PlayStation Store.
3. Click the icon with three dots. (This step is required in PS5)
4. Select “Redeem Code”.
5. Paste your key in the Redeem Code Window
6. Click “Redeem” to finish the key activation process.

Here you can check how to redeem a PlayStation Network Key on the website: