To turn a map feature into a highlight you will first click on the feature you are wanting to highlight to pull up its edit options. Then select the notes option (the furthest left icon). In the notes you will see an option to ‘Show as highlight’. Check the box next to this option, click save, and then be sure to save your map. Then your feature will show up as a highlighted feature on your share map.
Tip: Be sure to add text to your notes. This will enhance the look of your highlighted features on your share map
See video below for a visual example.