Pages: underline header the full page width

[AlanWilson]

Pages 10.0 on OS 10.15.3 (Catalina): in previous versions I could create a border under the header the full width of the page, to separate the header from the body of the document; now the border is only the width of the text entered. How can I create a border under the header the full page width, as part of the header (ie gets repeated on each page)?