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How can I accomplish these objectives?
First of all, the package flight function should be the answer for your question. You can use package flights to distribute specific packages to a limited group of testers. With package flights, only the packages are different; the Store listing details will be the same for all of your customers. Anyone in your flight group will receive the packages that you include in the package flight, while customers who aren't in the flight group continue to receive your regular (non-flighted) packages. So you could test your new feature within a small range.
But the package flights function is only available for normal MSIX package submissions, not win32 apps. I've confirmed this with team. So if you want to use the package flights function, you have to publish your win32 application as MSIX.
What you need to do is package your existing win32 application using the MSIX package tool. Then create MSIX app submission in the store.
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Thank you, Roy. How do I contact the store?
For purposes of my testing, I can create a UWP app with an MSIX installer. However, when it comes to apps I'll really develop they'll all be Win32.