Hey Phillip,
To my knowledge there is no tool to create ellipse features in ArcGIS Online. There is an enhancement request in place to add the ability to create circle or ellipse features.
If you add a sketch layer to your map, you will have the ability to create circles that can in turn be stretched into a ellipse shape but you won't have the ability to define coordinates as described in the ArcGIS Pro documentation. Sketches are different than features. They are simply shapes that reside in your map. You can also enable pop-ups on shapes and manually add some information to the shape that can be shown in a pop-up.
