1. Log into your Webmail account through the Plesk control panel (Click here to access the guide for help doing this)
2. Click on the settings page on the left had side
3. Click on the Identities option, then select the email address you wish to add a signature to. Once clicked, on the right hand side, scroll down to the signature editor. You can begin creating a signature in there. To enable the HTML editor, simply click on the image icon located at the top left hand corner of the signature editor panel.