To set a specific custom button on “Upcoming event” shortcode of a page builder, please first enable it in the shortcode pop-up options.
You can change label (i.e. Attend), specific URL (page on your site or another one – meetup.com, i.e.) and of course where to open this custom link. If you use service like meetup.com, we strongly recommend to enable “Open link in new window” option.