Hi Shawn,
In order to allow the Bing Map can be displayed on stand alone host Web Application Builder(WAB), There actually two ways to make it happened. The option depends on where's the audience of your application.
- For application only shared with your arcgis organization: Share the webmap in your organization/group, once the user sign in from the WAB application, the Bing Map will automatically display. The credential challenge is inevitable because that the way to get the Bing Map Key
- For application shared with public: You have to have check "Allow anonymous access to your organization's website. (ess.maps.arcgis.com)" in security setting in ArcGIS Online
Then in the "Map" setting check "Allow this Bing key to be used in maps shared publicly by organization members". Then for your localhost application you will able to see Bing Maps.
- The reason you can view Bing Map in ArcGIS Online or Web App Builder (WAB) is because you have already log in with your organization credential. For localhost WAB application, since you shared the webmap as public, you need to allow anonymous access to view the Bing Map in your organization, otherwise you will not able to see the Bing Map based on the arcgis online documentation: Configure security settings—ArcGIS Online Help | ArcGIS
PS: There is another way to manually change the code behind the WAB to hard code the Bing Map Keys in file BaseMapGallery\Widget.js. However since Bing Map Keys are charging credits for usage, we don't recommend this workflow.