app.beginUndoGroup("Loop Layer");

var myComp = app.project.activeItem;
var selectedLayers = myComp.selectedLayers;
var myLay = selectedLayers[0];
myLay.property("Effects").addProperty("Slider Control");
myLay.property("Effects")("Slider Control").name = "Loop Point";
myLay.timeRemapEnabled=true;
myLay.property("Time Remap").expression = 'framesToTime(timeToFrames(time)%(effect("Loop Point")("Slider")+1))';

app.endUndoGroup();