To create a return mechanism for an engine start switch, when the switch is held in the start position until the engine RPM is raised to the sustained level (like B737), you can use the "Starter_Return_#" parameter. It can be assigned as a simple digital output for an electromagnet or as a servo output, if this is preferable to you.
This parameter is associated with a special custom SimVim dataref which is used as an output for the actuator in accordance with each specific airplane model currently loaded into X-Plane.
Thus, this Start_Valve_Eng# parameter can be used both for a servo motor and for an electromagnet, and it should work correctly for each aircraft, you do not need to worry about finding special custom datarefs for it.
When you turn the switch to the GRD start position, it is held in this position mechanically, until the engine speed is raised to the predefined flight model start value and the actuator returns the switch back to OFF(AUTO) position.
1. Assign the "Starter_Return_1" (for engine #1) parameter as servo output, then calibrate servo in the plugin menu, as described here.
2. In the "Calibration Tool" window: select assigned servo, then move the servo to the "switch free" position and enter value = 0, then move it to the "push the switch back" position and enter the value = 1.
If you use a solenoid/electromagnet as actuator, you need to assign the "Starter_Return" parameter as a simple digital output to control the related driver or relay.