Disclaimer: Please contact your account manager if you wish to use this feature.
You can choose to make the email preheader section editable when you are creating Campaigns.
This is useful if you want different preheaders per Campaign. You can access this setting by the home navigation screen by selecting:
Settings > My Settings > Email > Email Preheader Editable.
Once the setting has been enabled, you can edit your email preheader by clicking into the section when in the Campaign builder.