You'd have to show exactly which bike and exactly which system it has on it (model and part numbers, brand, year, etc), because there's many different systems out there. (we still might not know, but it's unlikely for us to be able to tell with just the info provided so far).
Some of them are designed specifically to not have throttles, depending on the area they're sold in, and the firmware / settings of the controller won't accept input from one even if you add a connector to the place on the board inside it might have for one.
If yours is like that, you'd have to replace the controller (or whole e-system, if it has a display) with one that does accept a throttle.
Some might have alternate firmware you can "hack" onto the controller that let you use one anyway.