/D=C:\Bla or /D=C:\Path with spaces
Set installation folder ($INSTDIR)
Must be the last parameter on the command line and **must not contain quotes even if the path contains blank spaces.**
Default install dir is chosen by nsis installer if directory is quoted, which leads, on a x64 system, to an agent deployed to C:\Program Files (x86)\ whatever directory you choose (and plugins deployed to C:\Program Files\, which is the default directory chosen by the agent deploy tool)
The destination directory for agent installation should not be quoted as seen in NSIS documentation http://nsis.sourceforge.net/Which_command_line_parameters_can_be_used_to_configure_installers
Default install dir is chosen by nsis installer if directory is quoted, which leads, on a x64 system, to an agent deployed to C:\Program Files (x86)\ whatever directory you choose (and plugins deployed to C:\Program Files\, which is the default directory chosen by the agent deploy tool)