Lightbox on post thumbnail

Hi,
I tried to add the lightbox functionality to the automatically added thumbnail picture in a blogpost.

I found the following in the documentation (https://github.com/archetyped/simple-lightbox/wiki/Activation), that I need to add the provided code to the theme’s post loop.

Which filter/action/hook do I need to use in my functions.php file to apply the provided code?

Thanks and kind regards