When you send an email, your recipients see your name, the subject line, and the pre-header text.

The pre-header text is also called Johnson Box: the term commonly found at the top of direct mail letters, containing the key message of the letter. The purpose of pre-header is to draw the reader’s attention to this key message first and hopefully grab their attention, enticing them to read the rest of the letter.

So it is important to choose good pre-header text, so recipients are more likely to open your campaigns.

HTML designed email templates (for marketing emails)

You can change your preheader text in the HTML email designer by selecting Body and then editing the Preheader Text field:

Text-only email templates (for business / sales emails)

The preheader text is just a hidden <div> that needs be at the very top of the email.
So just add the following code to your text-only email templates editor:

<div style="font-size:0px;line-height:1px;
 Add 85-100 Characters of Preheader Text Here