app.selection.texts.overprintFill = true;
I would also consider setting texts.overprintStroke to true.
In case someone is a bit experimental with some text design.
contents of a text frame does not define formatted text.
And contents is no method or function.
should result in an error message: "my_FOLD_TextFrame.contents is not a function".
And then it does not make sense to go on and add:
my_FOLD_TextFrame.contents(0).overprintFill = true;
before resolving the problem.
Do as Chinna suggested and maybe add another line for overprintStroke as well.
I think this is great.
And what I end up doing is setting Paragraph to overprint:
myParagraph.overprintFill = true;