Custom widgets have to be added to your WAB following this help page guidance
You can get the eSearch widget at this link:
to view other custom widget or ask questions about custom widgets go to this group
Rebecca's extensive resource list for all things WAB:
Tapas's blog on installing eSearch and other custom widgets:
Be careful to pay attention to the space you are answering a question in. This is the Web AppBuilder space and ArcGIS Desktop has not relevance here.