How to show a lightbox popup when clicked on the button in header?

For creating a popup template kindly refer to this documentation and video tutorial: https://docs.codex-themes.com/article/229-general . After creating your popup, proceed as follows:

a) if you have built your header with the header builder, simply edit your header template and assign the onclick popup to the button as described here https://docs.codex-themes.com/article/235-on-click-popup

b) if you wish to assign this popup to the button in built-in header (top area), use this setting in Theme Options – Menu & Header