Add the menu item to a WordPress menu that you want to trigger the popup form. Then visit the site and find the unique ID of the menu item, you can do this by right clicking the item in your browser and clicking “Inspect”. Each menu item has an ID attribute such as
menu-item-X where X is a number.
Add the following code to the theme functions.php file (or create a plugin for it).
My menu item does not have an ID attribute
If your menu item does not have an ID attribute, but it has a unique class such as
page-item-34 then replace
.page-item-34 (note the hash symbol is now a dot).