You can set a global footer that will apply to all your marketing campaigns, but if you'd like to set a footer for one particular campaign, you can do so as well in the settings section of an individual campaign. To access the settings, first navigate to your campaign in the CAMPAIGN BUILDER section of the MARKETING icon and click on the name of the campaign.

From there, click on the GEAR icon toward the top right of the page. This will bring you to the settings page, where you can enter your additional footer information in the ADDITIONAL FOOTER box.