This video shows how to add a preheader in emails based on older templates.
For newer templates you will find this in the email settings section.
A preheader is the text you see right after the subject line when an email arrives at the inbox. By default, it is always the first piece of text in the content that’s displayed here, but marketers often tend to forget that it can be customized to either play nicely along with the subject line or present a short summary of what the email contains. After all, the subject line is your shot to encourage the subscribers to actually open the email, and the preheader gives you some extra space and words for higher open rates.