How do I a record a video on iPhone with the screen off?
Yes, it is possible.

This method works with at least iOS 9.


  
  Make sure your iPhone is locked
  Press the lock key on the top of your iPhone to brighten it, without unlocking it completely
  Slide the bottom right camera icon half way up, and don’t let go. Keep your finger in this way
  Slide the camera modes to video, again keeping your finger on the camera slider mode still half way up
  Tap the big red record button in the bottom middle of the screen; again, keeping your finger still on the camera slider mode still half
  way up. Don’t worry if it doesn’t show the recording happening.
  Double press the home button three times,
  Keep your finger still half way on the slider, until the iPhone automatically turns off its screen. This will take about 10 seconds
  regardless of what settings you’ve kept in the auto lock feature
  That’s it. With the screen off, your iPhone is recording a video.
  To end the recording and save the video, simply unlock the iPhone
  


Source: https://www.entrepreneur.com/article/277209