You can activate Microsoft 365 with a product key by redeeming it through Microsoft's official website. Once linked to your Microsoft account, you can install and use the apps included with your subscription.
Open your browser and visit https://microsoft365.com/setup.
Sign in with your Microsoft account or create a new account.
Enter your 25-character product key if prompted.
Select your country and language if requested.
Follow the on-screen instructions to redeem the key.
Open your Microsoft account's services and subscriptions page.
Select the option to install Microsoft 365 if it is available.
Download and run the installation file.
Open Word, Excel, or another included app.
Sign in with the Microsoft account linked to your product key.
Check that you enter the product key correctly.
Make sure the key has not already been redeemed.
Use the same Microsoft account associated with your purchase.
If you receive an error, follow the troubleshooting instructions on Microsoft's official support website.