Selecting Branded domain in New Marketo Email Builder

Hi all,

We have created new emails in the new Marketo Email Builder which is great to use so far. The only issue is that we can't find where to select our branded domain. In the legacy email builder there is an option to select which email domain we would like to use (see below screenshot), however in the new email builder we cannot select this and it defaults to a domain we don't use as frequently. Could someone please help me figure out how to update the branded domain in the new email builder.