Microsoft details how to open Windows Feedback app in Windows 10 for Windows Phones
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As there is no Insider Hub on Windows 10 Technical Preview for Windows Phone yet, if you run into an issue with the OS, you need to let Microsoft know what they need to fix. In order to contact Microsoft without the Insider Hub, you only have two options (for now). By pressing the volume down button and power key simultaneously, you will launch the Windows Feedback app. You can also open the Windows Feedback app from the Settings list.
When you use the hardware buttons to open the Windows Feedback app, the app opens right to the section where you experienced the bug. So if you are trying to send a text message and your run into a bug, you can press volume down and the power button and the Windows Feedback app will open to the Text Messaging section, making it easier to look at Text Messaging-specific feedback content or submit your own feedback.
