The quick'n'dirty way is to type a nonbreaking space (Ctrl+Shift+spacebar)
at the end of the series of regular spaces. If the document will be kept and
revised, though, this will drive you crazy later because you won't see the
different kinds of spaces unless you display nonprinting characters (click
the ¶ button).
A better way is to place a tab stop where you want the end of the line, and
insert an underlined tab character after the last word.
These and more are described at
http://word.mvps.org/FAQs/TblsFldsFms/LinesInForms.htm