Under Copyright Law, copyright protection is provided to the writer as soon as the screenplay (or other copyrightable material) exists in a tangible form. In other words, once a screenplay is completed, copyright protection automatically attaches.
It is strongly suggested that on the title page of any screenplay that is completed that a notice of copyright be placed on that page in the lower left-hand corner which would consist of the following example (if I were the writer): “Copyright Michael Norman Saleman, 2017. All Rights Reserved.”
For additional protection, you should also register your screenplay with the Copyright Office (see the section entitled “Copyright Registration“) and register your screenplay with the Writers Guild of America.