Add HTML web opt-in form to header

Is there a way to add my mail chimp email newsletter signup code to the header on Customizr under the navigation bar?

I will style it with CSS to float left horizontally. I have the form HTML code, I just need to know how (and where) to add it to the correct PHP file and how to have this header style for all web pages on my site.

I have not created a child theme as of yet, but I will if you suggest doing so.

The site is https://heartsnest.com.