For others finding this forum post, starting with 4.0 the command for Windows is:
SpineSetup.exe /S
"C:\Program Files\Spine\Spine.com" --install yourActivationCodeHere
The first command installs silently, without prompting. The second command registers the activation code, so it doesn't need to be specified when Spine is run subsequently.
And for Mac:
/Applications/Spine.app/Contents/MacOS/Spine --install yourActivationCodeHere
