[Theme:Manifest] How to widgetize Header section??

I’m new to the business. So with a great difficulty, I’ve done what I could on my blog: jagannathrutwik.com using Manifest theme, which looks perfect for me.However, I just want to add a small Follow/Subscribe provision with a box for email address between Menu and Post-date. It’s easy to bump in a widget in between but I found that the Header should first be widgetized and then could I do it. But how do I do that without a functions.php file at all? (which I see from the internet is the only method to widgetize) Thanks a lot in advance…