Ordinarily anything that should appear on every page should be anchored to
the page header (see http://sbarnhill.mvps.org/WordFAQs/AnchorToHeader.htm).
Note, however, that this is appropriate only for a text box containing text
you want repeated on every page. If you want a blank text box to be filled
in differently on every page, then the header is not the appropriate place
for it. And if this is to be a protected form, you'll need a text form
field.