Make the first thing that appears in the Email Template under the <body> a “mktEditable” section. Add whatever other content or “mktEditable” sections you want beneath that first section (it’s irrelevant what else is in the template). Then, when you create an individual Email Asset from this template, double-click the first region in the email. Click the “HTML” button in the TinyMCE rich-text editor. Add a pre-header at the very beginning of the code like this:
<div style="display: none !important;">This is the pre-header. It should be ~100 characters long for best results as different clients will show different amounts of the pre-header…</div>
<div>This is the actual start of the email content here!</div>
When you click “Update” you’ll only see “This is the actual start of the email content here!” in TinyMCE, but the pre-header will be applied.
