The central part of the customized short link is a branded domain. It is a domain, which is associated only with your brand. A custom domain usually contains the name of your company or the type of business.
For example, ‘short.io’ is the branded domain that belongs and associated only with Short.io.
The primary purpose of a branded domain is to increase brand recognition and improve the engagement of the shared short links.
If you still don’t have a domain, you can buy it either via any domain registrar or via Short.io. When you purchase a domain via Short.cm, you don’t need to configure it. Short.io will automatically activate instead of you.
Note: Short.io sells domains for the domains’ price on the registrars.
Here's how you can add your branded domain: http://help.short.io/en/articles/4065811-how-can-i-add-a-domain