I don’t know how to make Windows recognize the .manager extension files so that the Manager WindowsApp opens them directly.
As a workaround it’s possible to extract the Manager-x64.appx file into a folder and run it from there, because it’s just a renamed zip file. Either rename the .appx extension to .zip, or if you have 7-zip installed you can extract directly. The “app” folder inside this new extracted Manager folder contains Manager.exe. Then when you click on a .manager business file you can browse to this Manager.exe file and open the business.
This will make updating Manager a bit more tricky as you’ll have to extract the new versions each time and tidy up the extracted folders. If you keep the folder name the same at every update it should work.
But maybe someone else has a proper suggestion.