By default, SimplyCast's email editor pulls the first line of text from your email to be the pre-header text. You can change this by inserting the following HTML code into the "Source" section of the text editor.
<span class="preheader">YOUR TEXT HERE</span>
First, add a new text block to the very top of the email campaign, above the header (you can delete the header if you so choose). Be sure to change the background of the text block to match the rest of your email. Open the text editor of the block you just added and locate the "Source" button.
Upon clicking the "Source" button, the code for that text box will appear. As the text box is empty, the source pop-up will also be empty.
Insert the code about at the very top. Be sure to change where it says, "Your text here" to whatever you would like the pre-header to be.