Pablo,
You can't. The designers only allow methods/properties to be added to the outermost container, in your case the formset.
You can add the method to the formset and call it from the form.
I don't use formsets. Why do you want to use a formset instead of independent forms?
> I need to add a method to a Form that is part of a Formset.
>I can´t find a way to do that, because all the new Methods I have created were placed in the formset, not the form.