Fields don't generally concern themselves with position on the page
(with the exception of the Advance field, but that's provided mainly
for compatibility with documents imported from WordPerfect).

The idea is to find the position on the page where the address should
appear, place a frame there, and put the address (which may be the
result of a field) inside the frame. The frame's properties include
its position with respect to the page or the margins.

--
Regards,
Jay Freedman
Microsoft Word MVP        FAQ: http://word.mvps.org
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You'll need to fold a piece of paper and insert it in an envelope. Draw
around the window and remove and unfold the paper. Measure the position of
the rectangle and insert your address fields in such a position that they
will display their content in the rectangle.