The first video that is sent is the content builder, that is for building a page.
On your case, the Content Dock is a widget so you need an HTML or shortcodes to it.
All the settings you need for Content Dock is right under X > Content Dock.
If you place a form that is created on MailChimp, then style that form on MailChimp end.
Use the Headline shortcode, if you need a headline. an Image shortcode if you need an image, and Button shortcode if you need a button in the Content Dock widget.
If this is still not clear to you, please provide us your site URL and login credentials in a secure note so we can also understand what code did you put on the Content Dock widget.