Create circle/rectangle/ellipse in Map Viewer

s.schuette_ct

I want to create a new Feature Layer in Portal for ArcGIS and then use Map Viewer to create circle/rectangle/ellipse as polygon, the same way I can draw them in ArcGIS Pro:

However I do find any information on how to do this directly within ArcGIS Enterprise.
The new Web Editor does not seem to have this functionality either.

EmilyGeo

A new suite of drawing tools have been recently added to Map Viewer in ArcGIS Online. These will be included in the next release of Enterprise (11.5) next month. 

In the mean time, you can bring the feature layer into your Pro projects for access to all the tools there.