One way to do this would be to edit the defaults for the page types that you're using. Doing these first 5 steps will make it so all pages automatically inherit the header extra content attribute.

1. Under "Pages & Themes", go to Page Types
2. For the Page type, select the "Edit Form" button
3. When you mouse over the "Basic" area, choose the "+" icon.
4. Under the Custom Attribute tab, select "Header Extra Content".
5. Now, the Header Extra content will be included for all new pages.

However, you'll need to do the following additional steps in order to have the same header extra content on all pages.

6. Under "Pages & Themes", go to Page Types.
7. For "Page", select the "Attributes" button.
8. If you did Steps 1 - 5, you should see "Header Extra Content" there. You can enter your content here, then save.

It should now use the same header extra content on all pages across the site. Do the same for any other page types that you're using.

Thanks! I go to try :)