If you're looking to add a signature for your mailbox through webmail you can add one by following these steps:
- Log Into Webmail
- On the left-hand side Click on Settings
- Click on "Identities"
- Select and Click on the mailbox name.
- On the right-hand side, you should see a blank box with "Signature"
- Type in your signature and click save.
You can then send out a test email to verify that your Signature has been successfully added to your outgoing messages.