How to Create Popup in Wordpress Without Plugin?

Best practice would be to use the Child Theme so the header.php file doesn`t get overwritten on subsequent updates.

no plugin but still reliying on a third-party tool. How come you are first search result on Google???

Dont have a Theme editor..

Not working.

If I add the code by the Code Snippet plugin, will it work?

Looks simple, and no added plugin is always a plus.

Thanks for the information :)