(ArcGIS API for JavaScript) Sample of creating a form in a popup and using applyEdits operation to make edits to the feature layer.

Sample.js

require([
    "esri/Map",
    "esri/views/MapView",
    "esri/layers/FeatureLayer",
    "esri/widgets/Popup",
    "esri/Graphic",
    "dojo/domReady!"
  ],
  function (Map, MapView, FeatureLayer, Popup, Graphic) {


    //Create a new map and set the basemap to Dark gray
    var map = new Map({
      basemap: "dark-gray",
      //layers: [featureLayer]
    });

    //set a new view and set the properties of the view
    var view = new MapView({
      container: "viewDiv",
      map: map,
      center: [-86.049, 38.485],
      zoom: 3
    });
	
	
	
	var fl = new FeatureLayer({
		url: "URl to a feature layer"
	});

	map.add(fl);
	
	//On click show the popup window with the lat long and set the content to the form function.
	view.on("click", function(e) {
		e.stopPropagation();
		console.log(e);
		view.popup.open({
			title: "Clicked location: [" + e.mapPoint.longitude + ", " + e.mapPoint.latitude + "]",
		});
		view.popup.content = form();
	});
	
	//This function sets up a simple form. Where the user enters a number then clicks submit to apply edits to the feature layer
	function form () {
		//Create the form 
		var container = document.createElement('div');
		var popField = document.createElement('input');
		var button = document.getElementById('submitData');
		popField.setAttribute("type", "text");
		container.appendChild(popField);
		container.appendChild(button);
		
		//Add an event listener on "click"
		button.addEventListener("click", function(e){
			//Set up the attributes
			var attributes = {};
			attributes["POP2000"] = popField.value;
			// Create a new Graphic with the geometry and the attributes
			var addFeature = new Graphic({
				geometry: e.mapPoint,
				attributes: attributes
			});
			
			//Once the feature layer has loaded onto the map, apply the edits to the feature layer
			fl.load().then(function(){
				fl.applyEdits({
					addFeatures:[addFeature]
				}).then(function (result) {
					console.log(result);
					alert("Successfully added new feature with objectId of: " + result.addFeatureResults[0].objectId)
				}).catch(function (err){
					console.error(err.code);
				});
			});
		});
		return container;	
	}
  });

Live sample: https://jsbin.com/pupuvurasu/1/edit?js,output