FeatureLayer inherits from GraphicsLayer, so you should be able to use the add() method. See http://help.arcgis.com/en/webapi/javascript/arcgis/help/jsapi/graphicslayer.htm#add.To create a point graphic to add, where pointX and pointY are the x and y from your form:
// Create the Point
var point = new esri.geometry.Point(pointX, pointY, new esri.SpatialReference({ wkid: 4326 }));
// Create a point image symbol:
var symbol = new esri.symbol.PictureMarkerSymbol("/Images/point.png", widthInPixels, heightInPixels);
// Set up the template for the info window.
var infoTemplate = new esri.InfoTemplate("Point attributes", "${*}");
// Create a graphic from the point:
var graphic = new esri.Graphic(point, symbol, infoTemplate);
// Add the graphic
featureLayer.add(graphic);
Substitute your own spatial reference ID for 4326.
