There are two flavors of remote switches, USB powered or battery powered.
The easiest is a powered sensor using MppSensor. The switch or sensor is connected to one of the I/O lines (watch the voltage!). A momentary or toggle type switch can be used, when the switch changes state it will send a message to AutomationManager. Use the device Properties to configure the PIN number to be used for the sensor.
A momentary switch can serve double duty as the control button (providing the ability to reconfigure the wifi). A toggle will need to be assigned to a different pin from the button or it will appear to the MppSensor the button is being held for a long period.
If a relay is available the MppSensor can be configured to have the relay follow the sensor value. This can be useful as an in wall off/on switch using a regular toggle light switch.
A battery sensor can be used to detect either a momentary switch or a transition on a fixed switch (e.g. a reed switch used for a security system).
A remote switch can be used with the MppSleeper configuration, the only extra hardware needed (in addition to the battery supply) is a momentary switch to short the ESP8266 reset pin to ground. When released the ESP8266 will reboot and send it's signal and battery state to the AutomationManager/WemoServer. If you use an ESP01 and remove the red power LED standby current is about 17uA and a pair of AA batteries will last 1-2 years.