In what ways would you expect it to be communicated?
New players are far more likely to be facing bots, and far less likely to know about the report function.
I've played through the new NPE a few times, at some stage you can face other players also playing through the NPE.
After your first match against another human would seem the appropriate time to have a short pop up tutorial showing players how to add other players as friends, and also how to use the report function.
This way the focus can be on encouraging the social aspect of the game, instead of the report feature itself.
For everyone else they could be shown the same pop up tutorial on logging in like other announcements.
Any less and you're talking about social media posts which just don't reach a wide enough audience.