You need to use Alt-F9 to display the underlying field code, which will look
something like

{ MERGEFIELD mergeFieldName }

then put the switch in there. If you try to put it in when you see the
"chevron" view of the field code it will not work.

You probbaly need \@"hh:mm"

--
Peter Jamieson - Word MVP
Word MVP web site http://word.mvps.org/