Kumar,
OK, so you are posting in the wrong GeoNet space then. There is a Web AppBuilder for ArcGIS space specific for WAB questions (I have moved this question there). As far as adding a operational layer via the main config.json (like in the Flex Viewer), this is not an option. WAB does not allow for adding layer directly to the app from an ArcGIS Server url like the flex viewer did (without using a custom widget). If you want to add a layer at runtime then you use the OTB widget called "Add Data" if you want to add a layer to the app during configuration of the app then you add it to the AGOL/Portal webmap or use a custom widget like the Local Layer widget.
Release LocalLayer v2.2 · cmndrbensisko/LocalLayer · GitHub