You can position any of the drawing stuff (lines, shapes, ...) absolutely relative to the page.
Select and right-click on the shape, and then from the context menu "Format AutoForm... > Layout > More...".

But it's easy to select and move those shapes accidentally. So it's better yet to anchor them in the header or footer.
If you need the shape on one page only, the header would need to be special to that one page though (... such as "different first page" header for the first page of the document or section, or a special section just for that one page).

Regards,
Klaus