To create a signature for your outgoing emails in the webmail, do the following:
Click Settings in the top right of the webmail interface.

Click Identities and select the sender-identity you want to change.

Add your signature in the Signature-field at the bottom.

Click Save to save your changes.
HTML-signature
If you would like to change the size, color or type of the font, you can check the tickbox for HTML-signature and use the buttonrow above the signature field to edit the text as you like. If you know HTML, you can also edit the raw HTML-code of the signature by clicking the HTML-button in the button row.
