Yes, you can. Let’s say you use “hodl.com” as your custom domain. Create a neon.page using your custom domain (with any slug) e.g. “hodl.com/xyz”. In the edit tab, open the top dropdown and choose toggle the switch for “Use this neon.page as Home page for this domain.”
Furthermore, you can also set an Error Page, which will be shown when people use your custom domain with a slug that does not exist (e.g. “hodl.com/thisslugdoesnotexist”).