If you have a MIDI keyboard, that’s the quickest way: if you go to the Note Input and Editing page of Preferences and set the Input onto kit or grid option to Use percussion map. Now if you press MIDI note 46, otherwise known as B flat or A sharp just over one octave below middle C, you’ll get a hi-hat note with the open playing technique automatically. A closed hi-hat is MIDI note 42, otherwise known as F sharp a couple of notes below that B flat.
If you don’t have a MIDI keyboard and you’re doing it via the computer keyboard, you’ll need to add the open playing technique manually. If you’re going to do this a lot, you might want to define a nice, short unique popover string so that you can input it quickly. Go to Engrave > Playing Techniques and select Open in the Unpitched Percussion category. You’ll see it has the popover text open: change this to e.g. ho (hi-hat open) and OK the dialog. Now you can make any note on the hi-hat open by typing Shift+P and entering ho into the popover.