As a developer you can make use of the Replay plugin inside of the Unity Engine to capture gameplay, to do so follow these steps:
Open Replay Settings. (Tools -> Luna Replay -> Replay Settings)
Make sure you are signed in.
Click on the Capture tab in Replay Settings & then enter Play mode in the editor.
When the game starts, you will see the Luna replay UI overlaid at the top of the preview.
You may play the game up until the point where you want the capture to begin. When ready to start capturing, press "Start recording".
Play the game.
When you have completed the gameplay you wish to capture, you can press to finish the record.
The UI will now allow you to enter a name for your capture.
Press save and wait for the confirmation dialogue.
You can now exit Play mode.
You can read more about this process here.
Alternatively if you are having trouble enabling capture mode please click here for more information.