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To publish an app or a game in the Microsoft Store, you need to open a developer account first. Here is the document: Opening a developer account. Please go through this document and make sure you have an active developer account first.
Then you could create a submission for your app or your game in the Partner Center. You will need to set the price and availability for your app, upload your package, and create store listings like descriptions and screenshots for your app. Here is the document: App submissions. You could switch to different document with the menu on the left side.
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