I found a workaround for getting the Verizon Message+ app installed on my Windows 10 PC. I went to one of my computers on the my network that has a working copy of the app. I right clicked on the Verizon Message app>more>share.. It opens the app in the Microsoft Store with a share screen, dismiss the screen and next to the launch click the ... then install on my devices. Pick the device you are wanting the app on and it will send to the downloads in the Microsoft Store app on that computer.
Hope this helps those looking for a way to sideload it.