Initially dictation will be disabled on your machine. To enable it, open your start menu, and press the settings button.

In settings, press the Privacy menu button in the main menu.

Once in Privacy, press Speech on the left-hand menu.

In Speech, turn on Online Speech recognition

Once this is enabled, to start dictating on any Windows 10 Machine, press Windows logo key +  H 

Whenever you need it to listen just press the microphone button.