You may be asked by an admin to record POV demos. To do that, start by opening the console in-game:
Type "record <filename>" (where filename is a name you choose, without brackets <>)
This will start recording the match from your point of view
To end the recording you can either write "stop" in the console or just leave the game.
The file is commonly saved under the folder path "Steam\steamapps\common\Counter-Strike Global Offensive\csgo".
You can then send this recording via email or attach it to a message in the support chat.