First, your printer may not handle it.
Second, you would put a blue rectangle in the header, behind text filling that area.

If you want text to appear in that area as in your illustration, that would be done using frames (and text that is white in color) in a style.

• Marginal Text by Suzanne Barnhill, MVP
• Frames and Textboxes in Microsoft Word
• Letterhead Textboxes and Styles Tutorial