This feature is available with some paid plans
A subdomain refers to the letters on the left side of a web address. The middle part is called the root domain and the characters at the end are the top-level domain.
As an example, let's look at the URL for the Bitly Help Center, support.bitly.com:
Root domain: bitly
Top-level domain: com
Use a subdomain as your custom domain
You can use a subdomain as your Custom Domain. This is an effective way to use a version of your website's domain as your custom domain. The Wall Street Journal is a good example of this: They use a subdomain, on.wsj.com, for their short links.
There's one exception to this: While www.your-url.com is technically a subdomain of your-url.com, there's a general convention that it should have similar (if not the same) content.
If you use a subdomain as your Bitly custom domain, make sure you configure your DNS record for a subdomain rather than a root domain.