How to Add Content on the Header Top Bar?

First of all, make sure that the Header Top Bar is enabled. How to enable the Header Top Bar?

From your WordPress Dashboard, go to the Appearance > Customize > Header Options > Header Top Bar section.
Underneath the Left Content/Right Content options, you can use the dropdown menu to select one from the Text/HTML.
If you choose Text/HTML in the Left Content/Right Content options, you can insert the content in the editor below. You can also switch between a Visual Editor and a Text Editor using the tabs labeled Visual and Text.
If you want to directly add a media file, you can do so from here by clicking on Add Media.