The custom domain is part of your unique links to the content you create in eMarketeer.
By default your domain name will be app.emarketeer.com
This will be part of all urls you generate in eMarketeer such as forms, landing pages, email tracking links etc.
Using a via-em.com domain as a custom domain.
This is the preferred method as it allows for the use of the https protocol. You may freely use unused subdomains from our via-em.com domain to brand the domain of your account. The sub domain must be free (not used by another emarketeer account).
Using your own domain as custom domain
If you want to use your own domain (your company.com) instead, you can do this after making changes to your DNS.
Read more about how to do this here.