While searching for something else (new thread) I randomly stumbled across exactly what I was looking for:
extern void On_Message(const char* message, float timeout_seconds, long long message_id);
First the function needs to be declared in whatever file you are going to use it.
int text_id = TXT_NONE;
char const * text = "1. Find a way to sabotage GDI's power.\n2. Gain access to the old Soviet helipads.\n3. Overwhelm GDI and re-capture our radar station.";
if (text != NULL) { On_Message(text, 35.0f, text_id); }
Above code will output exactly what's shown in the screenshot.
The border will be green regardless of playing GDI or NOD. Best results are with text_id = TXT_NONE. As seen, \n produces a new line for multi-line messages. This message will stay 35 seconds.