Those would be instructions for a system administrator with access rights to the internals of Horde. You can't get to them (or at least, modify them) as a regular user. If Horde's configuration information for an individual user doesn't offer what you're looking for, you could ask LP to do it (in a support ticket). This would probably affect everyone, not just you, so they may choose not to (unless they had simply forgotten to enable HTML on this server in the first place).
You want a Linux command line? If you haven't paid for "SSH", about the easiest way to do it is via a one-time "cron job". You set up a cron job for, say, 5 minutes from now (* for all other fields) and erase the cron job once you get email confirmation that it ran. It's not terribly convenient, but will do for occasional use.