How to add custom JavaScript to Azure AD B2C SignIn flow

I want to add custom JS to Azure AD B2C flow, but I can't figure out how to do this.

I read the docs I think 10 times, but still don't get it.

So far what I understand

1. I have a Sign up and sign in (Recommended) user flow
2. I enabled JavaScript enforcing page layout (preview) in properties
3. I selected 2.1.2 Page Layout Version for Local account sign up page (This is the version of the packaged content containing HTML, CSS and JavaScript provided by Azure AD B2C)

As far as I understand docs now I have the ability to somehow inject JavaScript into the sign-in page without using custom page content option.

The docs stated that to do so I need

Include this code snippet into your sign-up or sign-in template for a self-asserted page

Where and how can I do that?