About manipulating preferences for a sandboxed app, in my experience you
don't need to care that it's sandboxed for the purposes of
getting/setting prefs. Simply defaults write com.microsoft.rdc.mac
sets the key (eventually) in the container plist,...
"With OS X El Capitan, you can continue to use any of these methods to
select a NetBoot, NetInstall, or NetRestore disk image from which to
start up a Mac: Use Startup Disk preferences: Choose Apple menu > System
Preferences, then click Startup Disk....