Matt,
We don't offer such flexibility with our cloud users today. As a workaround, some users are hosting their own endpoints that redirect to Retool as a way of giving out custom urls to their end users. I understand this isn't the most ideal setup; You can follow along this feature request here, and we'll let you know when this is released.
http://community.retool.com/t/custom-domains-urls-on-cloud-retool/2157