We noticed that the ability to add layers via web url is not available in the Beta version. Will this be an added functionality in the future?
Yup this is planned to be added in a future release. For now you can use the URL parameters for adding services into the Map Viewer beta.
Use URL parameters to modify maps—ArcGIS Online Help | Documentation
As a workaround, you can also create an item from the Web-layer in your org and then add that item to the map.
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