AddPrefixToLayerName.js

var onRun = function(context) {
	var doc = context.document;
	var selection = context.selection;

	if(selection.count() == 0){
		doc.showMessage("Please select something.");
	}else{
		for(var i = 0; i < selection.count(); i++){
		  
			var layer = selection[i];
			var layerName = layer.name();
			var layerPrefix = "prefix_";
			
			layer.setName(layerPrefix + layerName);
			
		}
	}
}