Your ebike has a speed controller, You will need to find out the type of throttle you have, many have a throttle that has variable resistance. (measure the resistance of the throttle from off to full throttle, If you find out that value you can add a pot to your controller. Or you can use a motorcycle throttle stop, locks the throttle in one position but you can easily change the throttle.
In order to fix the speed, you will also need to look at the controller you have. Some controllers have a max speed setting, but many do not.
You might need to upgrade your controller to a controller that supports this functionally.
Good Luck with your project