i want to do like this, which mean during the serial monitor, i would like to insert "255" means max pwm... then the num "255: will eventually insert into one of the coding to initalize motor at pwm "255" pwm..huuuuu
You'll need to rewrite the Serial Monitor application, then.
The Serial Monitor takes whatever text you have keyed in, and sends it to the serial port.
Unless you intend to rewrite the Serial Monitor application, then you must rewrite the Arduino code to collect the string that the Serial Monitor application sends, and convert the string back to an int.