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MessageBoxForPlayers(filter, message)
Message - the same as in MessageBox.
Filter - a number indicating the player to whom the message is displayed. It is calculated using the formula 2^(player number - 1).
MessageBoxForPlayers(1, 'Maps/Multiplayer/Rome/text.txt');
I opened the map as an archive, added the text.txt file with the text to the specified path, started the game, and no message appears. I even tried this:
MessageBoxForPlayers(GetPlayerFilter(1), 'Maps/Multiplayer/Rome/text.txt');