resource - .h file:

#define CustomAlert 1000

resource - .rcp file:

ALERT ID CustomAlert
BEGIN
  TITLE "custom alert"
  MESSAGE "^1\n^2\n^3"
  BUTTONS "OK"
END

code:

    Char alertMsg[64];
    StrPrintF(alertMsg, "Your value is %d", yourValue);
    FrmCustomAlert(CustomAlert, alertMsg, "", "");

do NOT pass NULL value as param in FrmCustomAlert - always use "".