This article assumes you read this article and know how to run a live webinar with OBS. If not please read this first.
You can show any app in your desktop using Open Broadcaster's "Window Capture" source.
To do so, open your OBS app. In "Sources" click on "+" button and from the options, select "Window Capture". Then give it a name, and in the next popup, from "Window" select what you want to show the attendees. e.g. I selected my browser tab that I wanted to show.
You can resize the source in the preview if you want to just show a certain part of the app.
That's it! Now follow this tutorial to start your live webinar!