[Answered ]-Initial value in form's __init__ for the model with generic relation

I have found a nice answer to my question here:

If you have already called super().init in your Form class, you
should update the form.initial dictionary, not the field.initial
property. If you study form.initial (e.g. print self.initial after the
call to super().init), it will contain values for all the fields.
Having a value of None in that dict will override the field.initial
value

The solution to the problem was then just adding one additional line:

self.initial['content_object'] = form_value