If the web maps in Portal are not shared with "everyone", then it is expected behavior for them to prompt for a login. Also, if the data layers in the web maps are also not shared with "everyone", they too will prompt for a login.
Hope this helps,
ArcGIS Server Manager is not "aware" of the concept of web maps. There is no way to 'create' a web map in ArcGIS Server Manager from web service URLs. Web Maps are typically created in ArcGIS Online or Portal for ArcGIS.
What is the workflow that you're trying to do?
From my understaing its not possible to add all maps only using on server url,
you have to mentioned all map services rest url to use all.
Retrieving data ...