I managed to get it working. From the Ubuntu Application Launcher I right-clicked my bibliography manager, Zotero, and selected th menu item Add to Desktop. I launched Scrivener's Tools/Options... menu and hit the Bibliography/Citation Manger Choose button. I then pasted the following line into the file manager edit box:
I put the "$USER" text in this forum post. When I copy and pasted it it was my actual username. Now Ctrl-g followed by ctrl-o launches Zotero for me.
Running Scrivener version 18.104.22.168 beta (64-bit) on UbuntuStudio 1610, Asus laptop w/Intel Centrino 2 & nVidia.