In the dashboard, you can see the HTML script column. Fill it with code and drag it below the header. Just see my BLOG for example.
by doing try and error in html code, you will find the better way to understand html.. to be honest, I'm still using the classic mode rather than use widget style.
Go to your HTML code and find header...now under header Widgets =1 make it to 2. then go to page elements..you will see another page element below header. click on it..select HTML- add your code. done!
I just wrote an article about this very thing on my blog. Check it out: Hacking Blogger Beta kumiko xo