What glue should I use for glue-down engineered flooring installation?
There is no reason to glue down engineered hardwood floor to an underlayment.  This is just for initial aesthetics.  The glue will NEVER last (in a residential setting).  The only thing the glue will do is give you fits and make your install seem tighter.  Within weeks or months the glue will come loose and you will have a floating floor.

Nothing wrong with a floating floor.  But the fact is you should be gluing the pieces together, not under them.  The big boxes sell the generic engineered wood flooring glue.  Buy that and glue your pieces together, not under them.  The only time I have seen gluing to the floor to be an option is thin flooring onto concrete for commercial purposes.  The only thing I would glue down in someone home is vinyl sheets/squares.