@mpt and @robert.ancell can comment on presentation in gnome-software.
As for how to prompt, you could do something simple like @Lin-Buo-Ren suggested. Alternatively you could check to see if the interface is connected with snapctl get --plug :removable-media during a hook, then do something interesting with that (eg, touch a file and have your app see if it exists and let the user know how to connect the interface).
There are other snaps that do this. It is also surprising to me that polarr would have removable-media auto-connected and there are nice ways of letting the user know what to do. -1 to auto-connect.