Why do you need the form field to be in the header?

If your goal is to have the field display on multiple pages, there is an alternative approach. Place the field in the body of the document, apply a unique style to it, and then reference the field in the header by inserting a STYLEREF field.

On the other hand, if you just want the field to look like it is in the header area, place it in a frame (use the Insert Frame command on the Developer tab), anchored to a body text paragraph, and then position the frame "on top of" the header area.