Using Mail & Calendar app, how to add event to group calendar?

Using Mail & Calendar app, how to add event to group calendar?

In the Android App, it is showing events that were set up in the group calendar that I subscribe to.  However I can't seem to figure out how to add an event to the group calendar in the app. 

I can add them in the desktop web-client but not from the app in my phone. There appears to be a dropdown arrow in the form for adding an event in the app (has the name of my personal calendar) but clicking it does nothing.

So how can I do this?