Convince here not to do it!

There is no such things as a combo microwave oven and exhaust fan which
has an exhaust fan powerful enough for serious cooking.

In addition, the combo costs you valuable working space above your pots.

In my last two homes, first thing I've done is to rip out the combo unit
and replace it with a Broan 400 CFM exhaust hood. They're great, and
have been discussed in detail in this NG about five years ago, with
pictures confirming their vapor gathering ability.

-- Larry