The level of support of an e-bike is normally being set by up-down push buttons. This can often be done with your thumb, without removing your hands off the handlebars, see for instance the Bionx throttle:

However, such a throttle increases the distance between the grip and the brake lever or shifter. That's why I made a throttle that is mounted on top of the brake lever clamp so that no extra space is needed on the handlebar.

I use this throttle for direct speed control, where the speed is increased or decreased in steps by pushing up or down. Since you have to press the buttons very often with speed control, the used SMD Panasonic EVQ-P6DB35 buttons only need a small pressure.

 These are the parts:

 This is the throttle with the PCBs glued on it:

 Glue it to the brake lever with 3M VHB tape:

 Side view:

 The throttle internal wiring: