Map Viewer "Classic" pop-ups have an easily way to add Fields or Arcade Expressions.

The new Map Viewer "previously beta" does not include this option. It appears you just type a "{" and then scroll down the list that shows up. This is not intuitive; I randomly came across it. Please add back the little plus sign. Another option would be to add a help/info button to provide users information on how to use the pop-up editor.

My current workflow is just to make the edits in classic, and hope nothing breaks.