In order to activate your Product Key on Steam, first, you need to install the Steam Client Application. You can download it here
After you download and launch the app, follow these steps:

1. Sign in with your Steam logins, and select ADD A GAME as shown in the picture below. You will get 3 options, from which you need to select the Activate a Product on Steam option.
2. After the Product Activation window pops up, select Next.
3. Select the I Agree option since Steam Subscriber Agreement needs to be agreed upon in order to continue.
4. Copy the Game Key and paste it in the Product Code field, then click Next.

There are a few ways you can activate your EA key. Activate your game key by downloading the EA client app for PC. You can also activate it on the Electronic Arts website.
In case you are using the app:
1. Click on My Collection and click on Redeem Code.
2. Simply enter your game key in the activation field and click Next. Make sure that there are no spaces left before or after your code.

In case you're activating the product key on the EA website:
1. Log in and go to your Account Settings page
2. Click the Redeem Product Code button on the left side of your screen. Enter your key in the activation field and click Next.

1. Visit Gog.com and sign in to your existing account or create a new one if needed.
2. Scroll down the page and click on Redeem code tab or simply click here.
3. Enter your game code in the designated area and click the Continue button.
4. After that, you will be able to see what you get from the code and decide if you want to redeem or cancel it. In case you decide you do not want to redeem the code, click on the Cancel button.

Activation through the website
1. Visit epicgames.com and sign into your Epic Games account or create a new account if this is the first game you activate on this platform.
2. Once you're logged in, use this link to activate your game.
3. Enter your key and click Redeem.

Activation through the application
You can also activate your key through the Epic games application. This is how you do it:
1. Launch your Epic Games application.
2. Click on your account in the top-right corner and then click Redeem Code.
3. On the page that shows up, enter your key and click Redeem.

Activation through the Rockstar Games Launcher application
In order to redeem games on your Rockstar Social Club account, open your Rockstar Games launcher (you can download Rockstar Games Launcher here) and follow these steps:

1. Sign in with your existing account or create a new one.
2. Press on the icon on the top right side and then click the REDEEM CODE button which is displayed in the picture below.
3. The Rockstar Code activation window will pop up. Simply paste your game code into the designated area and click the CHECK button.

Activation through the website
Register or sign in to a Rockstar Social Club account (you can find the account page here) and follow these steps:

1. Click on the Use Rockstar Code button of any product (the same activation window pops up for every product).
2. The Rockstar Code activation window will pop up. Simply paste your game code into the designated area and press the Activate button.

There are several ways to activate an Xbox exclusive key. It can be done via the Microsoft redemption page, Xbox console, Microsoft Store, or Xbox application on Windows. Some keys can only be activated on the Xbox console.
How to redeem the key via the Microsoft page?

1.  Log in to your Xbox account on this page here.
2. After you successfully logged in you will need to go to this redemption page.
3. Enter the 25-character code, select Next, and then follow the prompts.

In case the Xbox key cannot be activated on the Microsoft page, it needs to be redeemed on an Xbox console or Xbox application on Windows.
How to redeem the key via Xbox Series X|S and Xbox One?

1. Press the Xbox button to open the guide, and then select Store.
2. Press the View button to open the side menu, and then select Redeem.
3. Enter the 25-character code, select Next, and then follow the prompts.

How to redeem the key via the Xbox application on Windows?

1. Press the Start button, and then enter Xbox into the search bar.
2. Choose the Xbox app from the results to open the app.
3. Select your gamer profile in the top-left corner, and then select Settings.
4. In the Settings section, scroll down a bit and select Redeem under Redeem a code.
5. Enter the 25-character code, select Next, and then follow the prompts.

You are able to redeem a PlayStation Network key either through the website or through your console.
Website activation
1. Sign in to 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 to open up a drop-down menu.
3. In the drop-down menu you will see the Redeem Code section.
4. After following all of the previous steps, you should be in the key redemption screen where you can enter your key with ease. Just make sure to double-check carefully if there aren't any mistakes!
5. You will be prompted with a confirmation screen of the product which is about to be activated.

Console activation (PlayStation 4)
1. Make sure that you're logged into your PlayStation Network account.
2. Go to the PlayStation Store.
3. In the navigation bar, please select Redeem Codes.
4. Enter your key in the provided field.