...
Consult your DNS provider’s documentation for specific instructions on creating CNAME records.
Set custom domain
Then To set your custom domain in External Share application on go to Global Setting page.
...
Go to the “Custom Domain” tab and enable custom domain.
...
The domain should contain more than 5 letters and/or numbers, and at least 2 dots. They should also start and end with a letter, e.g. customdomain.example.com.
...
Any External Share links created by users on your Confluence instance (new and existing ones) will now include your custom domain.
Change or remove the custom domain simply by clicking “Change”.
...