If you're not exactly sure what are "external sources", where and how to configure them, please follow to this article. If you wish to add external sources to your marketing campaigns, you'll learn the details here.
Once you have configured external sources, the option to add them to your templates will appear in the editor.
If you click on it, you'll have an option to see and add the content from external sources (RSS, Twitter, Eventbrite).
As you can see for yourself, we offer you several options how to present this content - as a bare link, headline or HTML fragment. Once you choose an option and click OK, the external source placeholder will be added to your template.
Similar to other placeholders, you can design external sources placeholders to fit your signatures; changing the font to bold or italics, adding colors or other changes will be reflected in the final signature as well.