Peter, there is no "Inbox" activity widget (an activity widget is a sub-program within an app). Most launchers - including the one I use (Nova Prime) - let users add such widgets to a screen by long-pressing on any screen, then dragging an activity widget to it. An icon is then shown, for example "EMail", and when I tap it, I go directly to my messages list. These activity widgets are meant as shortcuts to specific functionality within an app. Here's a screenshot (the "Activities" option can be seen on the top right corner of the image):
There is a "Inbox" standard widget, that is NOT what is missing, nor what I need. Thanks.