@silvabokis has the right code. I discovered that if you are using your form in a lightbox within an index you need to apply his code in the code injection space in the main settings or inside the site.region file if you're in developer mode.
This twist on his solution wouldn't work if you have various forms throughout the website that all need their own redirect page. I'm using this for conversion tracking and retargeted advertising, so I'm just redirecting every form submission to www.mybaseurl.com/# and inputting that as my "converted" url.