<esri:EditorWidget Map="{Binding ElementName=MyMap}" GeometryServiceUrl="http://sampleserver3.arcgisonline.com/ArcGIS/rest/services/Geometry/GeometryServer"/>
Does the EditorWidget show the template for that layer? If no, be sure to specify an ID for that layer. The EditorWidget ignore layers that do not have an ID. The EditorWidget's LayerIDs property is optional but if you have set this, be sure that it includes the ID of the FeatureLayer with Polygon geometry. Also when working with FeatureLayers with Polyline/Polygon geometries, GeometryService property is required.
Below is an example of minimal settings for EditorWidget:<esri:EditorWidget Map="{Binding ElementName=MyMap}" GeometryServiceUrl="http://sampleserver3.arcgisonline.com/ArcGIS/rest/services/Geometry/GeometryServer"/>
At the right-end of the EditorWidget is Add Options button "...", when you click on this and the Add Options popup is shown, check Freehand checkbox. Select a feature type in your polygon layer and begin drawing.