How do i create a dialog that opens when loading the home page

[starboy]

I am a beginner in web design but I can usually get what I want using css, html and WordPress plugins. I want to create a dialog that opens when loading the home page, then hides laterally after a few seconds. So I want the window to reopen again if the user clicks on it. For a visualization, this site does exactly. https://pupunzi.com
Is it only possible with CSS and HTML or should I learn to use javascripts?

If possible, could someone tell me the right direction to start?

Any help would be appreciated.

[Author]

Posts