Here is http://msdn.microsoft.com/en-us/library/windows/apps/hh779720.aspx which provides more details about the Lock screen apps and other features. You may refer to the User management of lock screen apps section specifically for your situation.
Hi,
As Holger mentioned, you can create a shortcut and copy it to the Windows startup folder.
I have tested it works.
The steps for your reference.
1. Create a Shortcut on desktop.
2. Type the location of Lync App.
%windir%\System32\cmd.exe /c start "" "lync15mlx:"
3. Type a name for the shortcut and then click Finish.
4. Copy the shortcut to the Windows startup folder.
%UserProfile%\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup
Best regards,
Eric