There are probably at least 10 topics here describing it, not sure what you searched for but no worries.
You can when editing menus in wp click the Screen Options tab top right of the screen and check the CSS class checkbox. Now add/edit a menu item and add the emodal class for the correct modal.
Hope this helps. Please take a moment to rate and review the plugin and or support https://wordpress.org/support/view/plugin-reviews/easy-modal
Be sure to set the modal to load sitewide.