Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Do you mean Product Key for Microsoft 365 subscription?
If so, you shouldn't need a product key. When you first purchased from third party, they gave you the key code, but it was for redeem purpose, so it can link to your Microsoft account. Once redeemed, that code is considered used, moving forward, your Microsoft account serves as your Microsoft 365 product license.
So, you can just go ahead and install.
Please go to https://account.microsoft.com/services
Sign in to the Microsoft account you used for the subscription.
You should then see your subscription listed. Find the option called "Install desktop versions of Microsoft 365 apps like Word, Excel and Powerpoint to your devices". It has a down arrow icon next to it.
Click on that. It should start downloading the installer file. Then you just need to run that installer file to install the apps.