The Microsoft Dynamics Marketing instance is not named after the default domain from the Office 365 tenant.
The Microsoft Dynamics Marketing URL naming convention is currently designed to name the instance after the primary Office 365 domain or the first added vanity Office 365 domain.
If the Office 365 tenant that was used for the Microsoft Dynamics Marketing provisioning process has only a primary domain, then the instance will be named after the name of the primary domain. However, if the Office 365 tenant has also vanity domains (secondary domains that are accepted by Office 365), the Microsoft Dynamics Marketing instance will be named after the first added vanity domain.