Is there a way to have the map open with the legend open?
Yes, add a map legend as a separate element:
https://doc.arcgis.com/en/dashboards/latest/get-started/map-legend.htm
You cannot have the map legend within the map element on by default.
@ChristopherCounsell Thank you! The only issue with adding it as a new element in a dashboard is that on a mobile device it looks horrible.
I would recommend not having a legend on by default if your audience will be mobile. The in-map legend should be fine for the majority of users; most people who can navigate a mobile phone have seen some icons on maps and websites. You can also provide a user guide for basic functionality, contact details...
If users are struggling with identifying features you might be able to redesign the map. Less layers, labels and symbology often helps significantly. I try to design the maps so that you don't need a legend, where possible...
So the technical answer is no, you cannot have the in-map legend on by default, but there is a dedicated legend element available.
The design answer is consider the purpose of the application, your audience and their devices. Can you adjust the symbology? Add labels? Share a user guide for tools/context/contacts? Remove unnecessary layers and elements bloating the app?
some documentation here....