There are 2 methods to include Onclick popup inside the menu link.
1) Using standard WordPress method
- Go to your WP admin panel
- Go to Appearance->Menus
- At the upper right corner of the Page you will see "Screen Options" DropDown menu, click on it and check "CSS Classes" option.
- You will see CSS Classes (optional) option
- Add our class 'sg-popup-id-1' where id-1 is a popup id.
2) Using 3-rd party shortcode plugin
You can use 3rd party plugin which will allow you to use shortcodes inside the menu item.
Please follow the steps to get it done:
- Install the following plugin https://wordpress.org/plugins/shortcode-in-menus/
- Check this screenshots https://wordpress.org/plugins/shortcode-in-menus/screenshots/ to activate shortcodes panel
- Add our shortcode like this [sg_popup id="1" event="click"]Your text[/sg_popup]