Manon is a French female firstname. There’s a novel by Abbé Prévost called Manon Lescaut, which was used for an opera with the same name by Giacomo Puccini.
There’s also a Welsh first name “Manon” but I have no idea how to pronounce Welsh. So let’s talk about the French name:
The “a” is a short a like in “part” or like the u in “hut”. The second n is very french and there is nothing comparable in English or American language. It is a nasal vowel. To produce it, lower the soft palate so that air escapes through nose and mouth at the same time.