1. Visit the URL http://webmail.<domain_name> and login into horde mail.
2. Hover on gear icon, then preferences and click on Mail option.
3. Click on Personal Information from the General section under Mail option.
4. A. Now see below on that page you will find Your Signature text area. Type your signature as text input.
B. You also can put your HTML Signature as shown field next to it.
If want preceding dashes, then check the marked option and click Save button.
5. Now go to Compose section under Mail Settings and click on Composition.
6. Mark the option as show the signature on the compose screen and click Save button.
7. You can see your text signature when composing any mail as text mail.
Also can view the HTML signature when composing as an HTML mail.