By default, MyListing allows you to choose a page (Theme Tools > Theme Options > Header > Show Call to Action Button > Call to Action Links to Page) that the CTA button links to.
This code snippet allows you how to link to an Elementor Popup, and the Open an Elementor Popup From the MyListing Header CTA Button guide will help you implement the overall solution.
jQuery('.header-right .header-button a.buttons').attr({
'href':'#addlisting',
});