When configuring a custom hostname in the Splash Page URL, it's required to create a CNAME.
In order to do that, you need to go to the website where you registered your domain (i.e. mydomain.com) and create the CNAME through the control panel of your domain management section.
Instructions might differ depending on the specific service used to register your domain.
As an example, the following video shows how to do it with GoDaddy
