You should take a look at the client documentation (available on https://apidocs.mendix.com). To execute a microflow from a widget, use mx.data.action() (see here).
This is also covered in the MobileSlider widget tutorial https://world.mendix.com/display/howto6/The+Mobile+Slider+Custom+Widget
mx.ui.action is prefered above mx.data.action as will set the origin and allows you to use the progress indicator
https://apidocs.mendix.com/6/client/mx.ui.html
mx.ui.action("MyFirstModule.PetCat",{
progress: "modal",
params: {
applyto: "selection",
guids: [ this.contextObject.getGuid() ]
}
});