The Submission Completed Processor runs on the server, so it won't be able to perform actions on the form directly. Are you trying to redirect the submission landing page dynamically after the submission? You can achieve this by pointing your submission confirmation page config in TM to a submission property, and setting the value of that property in your SCP.
If you are trying to open an additional page when the form is submitted, you are better off running a dynamic data service that gets triggered by the submit button (either before, or in the 'onSuccess') event.
Thanks for your help!
I wanted to open an additional page. I got that working using Util.openURL() on the click of a submit button('onSucess' event)