Not sure what you're asking. As your menu shows, it's just a matter of toggling it on or off.
If you meant to ask "how do I make an object overprint", that depends on the object and the effect you want. Simple overprints are assigned in the Attributes panel (under View). You can make a fill or stroke overprint by checking the appropriate boxes. Other methods of overprints can occur depending on a Transparency Blend mode, like Multiply.
I need to set it through JSX? In the question I asked above, i forgot to mention it. Apology
Window >> Attributes.
I though for many years prefer to use Window >> Transparency >>Multiply.
I might be misinformed, but Multiply and Overprint are not quite the same thing.
Overprint and Darken look more like the same result. Darken has the advantage that it could also be used when Overprint fails, for example when a spot color is converted to process.
They are definitely different animals. Depends on what you want to accomplish
They are different animals, but they will create the same separations and Darken is a nicer animal that behaves better when spots are converted to process.
Darken does not need Overprint Preview, faster preview and better support in other apps (like Apple Preview and non Adobe PDF viewers).
If you want a similar result as Overprint, Darken is the way to go, but yes, Multiply can give effects that Overprint cannot, and still separate correctly.