As a Roblox developer, it is currently impossible to provide an in-game report method for a player on a different server.
With MessagingService, you can have real-time communication between two servers.
Player A does something against the terms of service to Player B, which is in an entirely different server.
It is impossible for Player B to report Player A from the in-game menu, or using in-game methods.
Additionally, if the player reports the user from the website, all messages that the player sent using MessagingService only contains their messages (if filtered properly), and not the target players responses.