The strange character was used in the title of a 18th century French print. The title is quoted in a journal article to be published next year.

Late last night I was told by the publisher: " with X-Press I can use a "floating" accent and produce an m+tilde". So, that's that.

p.s. If I have the time and inclination I will try to figure out why the field code solution did not work. (I wonder if it is an issue with my keyboard and/or the OS.)