You can create an email signature via Webmail. All you need to do is log in to your email account using Webmail, then navigate to Settings → Identities section:
The area to add the email signature will be displayed on the bottom. You can also click on the image icon to open the format options. Once you've created the signature, click Save:
That's it! From now on, the newly created signature will be automatically added to all of your messages.