The channel is the last parameter in the function set_(d)hudmessage. Set it to "-1" to make it automatically detect an available one.
PHP Code:
set_hudmessage(red = 200, green = 100, blue = 0, Float:x = -1.0, Float:y = 0.35, effects = 0, Float:fxtime = 6.0, Float:holdtime = 12.0, Float:fadeintime = 0.1, Float:fadeouttime = 0.2, channel = -1);