Testing methods post on the form using django-webtest

I want to test my form using django-webtest. I want to create an object (make post) using the form. But I get the error on the amount of:

my_form.form['country'].value = "USA"

My all test:

def test_post_view(self):
    my_form = self.app.get(reverse('myapp:add_page'), user=self.user)
    my_form.form['country'].value = "USA"
    my_form.form['name'].value = "Christmas"
    my_form.form['date_from'].value = timezone.now()
    my_form.form['date_to'].value = timezone.now() + datetime.timedelta(days=5)
    my_form.form.submit()

My form:

class MyInputForm(forms.ModelForm):
    class Meta:
        model = MyModel
        fields = ('country', 'name', 'date_from', 'date_to',)