Django Set foreign key field in model from the url parameter

Question

I have a foreign key field in my model which should be set from the url params when the model is saved.I need to set the field automaticatically from the registration-id

models.py

class Mymodel(BaseModel):
    name = models.CharField()
    created_by = models.ForeignKey(
        'User',
        related_name='',
        null=True,
        blank=True,
        on_delete=models.SET_NULL
    )
    last_edited = models.DateField(auto_now=True)
    registration = models.ForeignKey(
        SomeModel,
        related_name='',
        null=True,
        blank=True,
        on_delete=models.SET_NULL
    )

views.py

class MyCreateView(LoginRequiredMixin, CreateView, CustomPermissionMixin):
    form_class = ''
    pk_url_kwarg = ''
    template_name = 'c.html'

    def form_valid(self, form):
        form.instance.created_by = self.request.user.person
        return super().form_valid(form)

    def get_success_url(self):

        return reverse('')

forms.py

class MyModelForm(forms.ModelForm):
    class Meta:
        model = Mymodel
        fields = ('name',)

urls.py

'<int:registration_id>/employee/create/'