The Popup in your reference example
http://cultivateevent.org is
sticky,
animated (slides in from the right),
sits off-page (so it stays to the right hand side of the browser) and has
multiple links,
close popup and
links to another page.. There is some slick coding in that lot!
You need two Xara work-arounds; one that allows sticky animated pop-ups and one that allows that popup to auto-load. Attached is a working example with some explanations that you can have a play with.
You can use the approach across multiple variants, but for a mobile variant I would simply forget the popup and have the registration block always visible on the page.
Have fun
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