You cannot find it in the glyphs of a regular font because it is not in the official Unicode set (a font may still contain it as a precomposed character, but that's very rare).
Using a font that has both an "m" and the spacing or nonspacing tilde accent (U+02DC or U+0303): type the 'm', type the '~', use manual kerning between the two to move the tilde on the center of the ''m', then use manual kerning after the tilde to make sure the next character follows properly.
Alternatively, create a custom font containing this character only inside InDesign
using my own IndyFont: Indiscripts :: IndyFont | Build OpenType Fonts from within InDesign!
Thanks Jongware. I can kern the tilde to the centre of the m but how do I move up the tilde to the top of the m?
You can use baseline shift on the tilde, but if it you use one of the glyphs recommended by Jongware rather than typing it from the keyboard you may not need to..
Right -- the ASCII tilde, as inserted 'from the keyboard', may be a bad choice because it usually is designed to be much larger, wider, thicker, and generally meant to insert between text (just like an en-dash), rather than be used as an accent. If the font contains designed spacing or nonspacing accents, use these instead.
This is an interesting challenge.
I just tried to locate/enter the U+0303 from the glyphs panel, not really fun.
A simple way would be the following script, assuming an insertion point selection:
app.selection.contents = "m\u0303";
Then you have to find a font supporting that combination. Minion and Myriad failed, but both Arial and Times New Roman looked ok, even though the latter places the accent at a higher position than for then ñ.
I don't know Vietnamese but they use plenty accents all over the place, so looking for fonts that are prepared for Vietnamese may get you further.
Sorry I had to go out without thanking the various people who have helped me. I was able to do it okay with Jongware's method and further info from Peter Spier. Thank you all.