You can create an email signature for your messages via Webmail. All you need to do is log into your email account using Webmail, then navigate to the Settings → Identities section:

Scroll down to find the area to add the email signature. Optionally, you can 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 your messages.