Pressure radio are using the ability to pass a timezone code to the plugin to display events in the user’s chosen timezone. They offer a few preset links, as well as a complete dropdown list.
If you want to do something similar, simply add the necessary html links or in form html before the shortcode.
WordPress itself has the html required for the timezone list in the Settings> General tab (use inspect element), or some form generators will create a dropdown form of timezones for you. Links would like url?tz=timezonestring