where do I find the Key for my MS Office App?

The product ID number shown in the About menu for any version of Office is not the same as the activation key.

For Office 2011 and earlier, the product key is on the back of the sleeve the installation DVD is in. Can't remember if Office 2008 was ever available as a download purchase, but Office 2011 was. So, if you purchased Office 2011 (and maybe 2008), the product key would have been sent to you in an email. If you've lost either the original disk, or the email, you're out of luck.

Office 2016 also came as a boxed version or download. The download purchase would, again, have generated a confirmation email with your product key. If you purchased the boxed version, all that's in the box is a web site to download the installer from, and your product activation key. Both of these also fall under the simple rule of - if you lost either, you're out of luck.

If you're referring to Office 365, I can't say how that subscription software works, but I would imagine that would also be an email for your activation key.

Try the Word ➙ About Word menu option. The Product ID key should be in that window:

You can try asking on the Microsoft help forums: Microsoft Community