how to setting additional attributes of a join model?

I've spent a lot of time trying to create a new model and, at the same
time, setting an attribute of the join model.
I have:
Company
has_many :managements
has_many :managers, :through => :managements

Manager
has_many :managements
has_many :companies, :through => :managements

Then I try to create a new manager with a associated company like this:

- args = @company.nil? ? [@manager] : [[@company, @manager], {:remote => true}]
= simple_form_for *args do |f|

  = field_set_tag t('manager.manager') do
          = f.input :name
          = f.input :surname
          = f.input :fiscal_code
          = f.input :city
          = f.input :zip_code
          = f.input :address
          = f.input :street_number
          = f.input :tel
          = f.input :email
          = f.simple_fields_for :managements do |p|
            = p.input :role

I don't see the field role in the form and I don't know how to set
role when the Management model is created.
How to do this?
Please an advice.
Thanks.