In Web AppBuilder, there isn't an out-of-the-box way to add tabs to the web app. However, this is a functionality that is available through Operations Dashboard (Dashboard layout—ArcGIS Dashboards | Documentation) and Story Maps Series or tabbed StoryMaps (Esri Story Map Series - Present a Set of Maps and Other Content). It may also be possible through customization in Web AppBuilder for Developers. If you'd like, you can also look into logging an enhancement for this functionality on our Ideas page https://community.esri.com/community/arcgis-ideas. For more information, I'd also recommend reading the GeoNet post here: Web tabs in web mapping application. Hope this helps!