To do this, log in to your Horde webmail client and click the [New message] button. In the "Message Composition" window that opens you will see the option [Switch to HTML composition]. Click it and a WYSIWYG editor will open, giving you the option to edit your text's font, size, format, to insert links and images, etc.
Once you are ready with the message, click [Send Message] to send it.
NB: The option for [Switch to HTML composition] cannot be displayed properly by some browsers. If you don't see this option available for you, try using an alternative browser (confirmed to work with Firefox).