You can use our Custom Elements capability to visually add/edit preheaders in email templates. You can use the Elements tab to register custom blocks in an email template and then subsequently visually edit rich text areas, images etc.
Simply add this markup to any HTML email template to be able to get a text area in Elements for editing:
<span id="ad_text0" class="preheader" style="color: transparent; display: none; height: 0; max-height: 0; max-width: 0; opacity: 0; overflow: hidden; mso-hide: all; visibility: hidden; width: 0;">Type the preheader text</span>
You will then see a text area under the Elements tab, where you can type in the preheader.
Once an email is sent, it should show in your inbox like this with a preheader:
Additional details on how to use the Elements tab to register custom blocks in an email template can be found at: https://help.blueshift.com/hc/en-us/articles/115002708433-Email#Editable