You never did this from the guide:

#include "DesktopNotificationManagerCompat.h"

It's that one because I downloaded it under that name as mentioned in the description:

#include "gwin32notificationbackendprivate.h"

but you said gwin32notificationbackendcompat.h contains only a single function declaration. MS don't want you to include that: they want you to include the contents of DesktopNotificationManagerCompat.h as they provided. If you don't, it's probably not surprising that you get errors about undefined things, which their article probably expects you will have had defined by what they told you to include.

To be clear: I am suggesting to include DesktopNotificationManagerCompat.h in whatever 3rd source file you are implementing following the remaining steps (not e.g. assume that only its correspondingly named .cpp needs to include it)

I used the wrong file name in the description, it was meant to say gwin32notificationbackendprivate.h. I corrected the description and my comment.