When updating a recent story map we wanted to include a mailto: tag but I couldn’t find an email link in the hosted story map tool. After some internet sleuthing I realized I was making it harder than it needed to be.
When editing your story map, click on the < > icon to see the source code in your editor:
Once there you can do most html editing that you need.
ESRI GeoNet https://geonet.esri.com/message/388988#388988