Thank you for reaching out to the Community! We do completely understand how important it is to edit your subject line and preheader. While you are editing those header options, be careful not too click out of the pop up window that appears when editing, or the option to edit will go away. Also, the preheader option does not appear for all email clients. When sending yourself a test, if you don't see the preheader appear, that means your email client does not have that option to show anything but the subject line.
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