I also needed this solution for a "kiosk PC"
Create a shortcut to your desktop or where you want.
The location of the item is:
ms-apps:///providers/Microsoft.PowerApps/apps/YOURAPPLICATIONID
And you can copy the application to a startup folder
To access the startup folders you should do the following
1. Press Windows key + R
2. Type shell:common startup (Startup for every user who logs in the machine) or shell:startup (Startup only for the currently logged in user)
3. Press Enter
If you don't know how to get the application ID go to https://make.powerapps.com/apps
Click on the "more button" than Details

There will be a section called "App ID"

(Tested on Windows 10 v1909)