If you have an ESC that is suitable for the motor you can use it without a potentiometer by connecting the potentiometer input to the terminal that would normally be connected to the clockwise end of the potentiometer. Check the ESC information to be sure that the CW end of the potentiometer is normally connected directly to a terminal.
You also need to check the ESC information to determine how the motor is started and stopped. Your start/stop relay must supply that command. It is unlikely that you can start and stop by connecting and disconnecting the motor to the ESC or the ESC to power.
It might be a good idea to start with the ESC connected exactly as recommended using a potentiometer. Make sure the motor runs well and then change the speed setting method.