is there a way to lock the scrivener app with a password so that no one else can access my WIP without my permission?
No.
Because of the structure of a Scrivener project, merely locking the application would not deny access to your work. Anyone with unrestricted access to the computer can simply use Windows Explorer to browse directly to the individual files.
If you need to secure your data, you should use one of the many available disk level encryption tools.
Katherine