This is an optional step to help with branding.
When your contacts hover of a link in an email they see the domain that link points to. When they click through to the email preferences page they see the URL of that page. If they see a domain they don't recognize they may not click that link (report spam even) or unsubscribe if they believe they are dealing with an unknown organization.
When you setup a custom domain with your Envoke account, reference to all Envoke branded domains will be replaced with your own domain
in tracking links (visible when contacts hover over links in emails)
This helps brand recognition for contacts and can even reduce unsubscribe rates.
Here's how the preference manager page's address looks like without custom domain
The same page with custom domain enabled:
You can use a subdomain of your primary domain as a custom domain: anything.yourdomain.com
Examples of custom domains:
Check your current custom domain
To determine if a custom domain is currently enabled for your account go the Domains section of the Account Settings page and check the custom domain field. If the field includes a domain (not blank and doesn't say "none) then a custom domain is already enabled.
Configuring a custom domain