which hook should I use

Hi there,

A popup screen like a live chat window?

If so you will need a find a plugin that does it.

mailchimp popup form. it's code is generated by mailchimp, some js code.

there are setting on mc like "when user scroll down to page", "after 20 sec" .

I'd like to embed it so that when user scroll down a blog post, it will popup.

js code should go into the wp_footer hook.