Log in to your Digital Ecosystem and select the "My Email Marketing Campaigns" tile.
Go to the settings tab, and then click on "Change Sender Address", from the pop up click on "+Add a New Domain".
Type in the domain you want to use, hit enter. Then you will be prompted to add some records in to your domains' DNS settings.
Once you have entered the setting correctly, hit the Refresh button, and the status for the records should turn green. Then click Done.
Your new domain name will appear as an option when creating a new sender email address.