scmJS docs

Display Text Message

Shows the text on the player's screen. Always Display shows it even to a player who has turned subtitles off; Don't Always Display follows that setting. The text can carry the game's colour codes.

Action 9 · Display Text Message(Display, Text);

Argument Takes Stored in the action
Display Always Display or Don't Always Display bit 0x04 of byte 28
Text text, stored as a string-table entry bytes 4–7
Display Text Message(Always Display, "Defend the base until help arrives.");