Pre-installed Microsoft Office not activating?

I've just bought  new Dell Inspiron 3501, and picked Microsoft Office 2019 Home and Student as an option; however, when starting any Office programs it tells me I either need to subscribe to Office 365 (which I don't want to do), or activate with a product key. I'm assuming therefore that I should have a product key to enter, since I paid for the full version, but I have no idea where it is? Any idea what I can do to get everything up and running properly?

Thanks in advance!