gistfile1.txt

# models.py 

def subscribe(self, plan, quantity=1, trial_days=None,
              charge_immediately=True, prorate=djstripe_settings.PRORATION_POLICY, discount_code=None):
    cu = self.stripe_customer
    """
    Trial_days corresponds to the value specified by the selected plan
    for the key trial_period_days.
    """
    if ("trial_period_days" in djstripe_settings.PAYMENTS_PLANS[plan]):
        trial_days = djstripe_settings.PAYMENTS_PLANS[plan]["trial_period_days"]

    if trial_days:
        resp = cu.update_subscription(
            plan=djstripe_settings.PAYMENTS_PLANS[plan]["stripe_plan_id"],
            trial_end=timezone.now() + datetime.timedelta(days=trial_days),
            prorate=prorate,
            quantity=quantity,
            coupon=coupon
        )
    else:
        resp = cu.update_subscription(
            plan=djstripe_settings.PAYMENTS_PLANS[plan]["stripe_plan_id"],
            prorate=prorate,
            quantity=quantity,
            coupon=coupon
        )
    self.sync_current_subscription()
    if charge_immediately:
        self.send_invoice()
    subscription_made.send(sender=self, plan=plan, stripe_response=resp)
    
# forms.py

class PlanForm(forms.Form):
    plan = forms.ChoiceField(choices=PLAN_CHOICES)
    coupon = forms.CharField(max_length=30, required=False)
    
# views.py 

class SubscribeFormView(LoginRequiredMixin, FormValidMessageMixin, SubscriptionMixin, FormView):
    # TODO - needs tests

    form_class = PlanForm
    template_name = "djstripe/subscribe_form.html"
    success_url = reverse_lazy("djstripe:account")
    form_valid_message = "Congratulations! You've successfully subscribed to Zana Edge. We'll be in touch shortly via email."

    def post(self, request, *args, **kwargs):
        """
        Handles POST requests, instantiating a form instance with the passed
        POST variables and then checked for validity.
        """
        form_class = self.get_form_class()
        form = self.get_form(form_class)
        if form.is_valid():
            try:
                customer, created = Customer.get_or_create(
                    subscriber=subscriber_request_callback(self.request))
                customer.update_card(self.request.POST.get("stripe_token"))
                customer.subscribe(form.cleaned_data["plan"], coupon=coupon)
            except stripe.StripeError as e:
                # add form error here
                self.error = e.args[0]
                return self.form_invalid(form)
            # redirect to confirmation page
            return self.form_valid(form)
        else:
            return self.form_invalid(form)

1. I believe the kwarg is coupon, not discount_code. see here
2. You don't seem to be doing anything with discount_code in the view. Pull it from request.POST just as the call to customer.update_card does on line 60 and then add it as a keyword arg to customer.subscribe (currently on line 61)

Oh, right. Like this?

class SubscribeFormView(LoginRequiredMixin, FormValidMessageMixin, SubscriptionMixin, FormView):
form_class = PlanForm
template_name = "djstripe/subscribe_form.html"
success_url = reverse_lazy("djstripe:account")
form_valid_message = "Congratulations! You've successfully subscribed to Zana Edge. We'll be in touch shortly via email."

def post(self, request, *args, **kwargs):
    """
    Handles POST requests, instantiating a form instance with the passed
    POST variables and then checked for validity.
    """
    form_class = self.get_form_class()
    form = self.get_form(form_class)
    if form.is_valid():
        try:
            customer, created = Customer.get_or_create(
                subscriber=subscriber_request_callback(self.request))
            customer.update_card(self.request.POST.get("stripe_token"))
            coupon = self.request.POST.get("coupon")
            customer.subscribe(form.cleaned_data["plan"], coupon=coupon)
        except stripe.StripeError as e:
            # add form error here
            self.error = e.args[0]
            return self.form_invalid(form)
        # redirect to confirmation page
        return self.form_valid(form)
    else:
        return self.form_invalid(form)

That will work, but you'll probably need to specify coupon=coupon or else that second kwarg might be interpreted as quantity

Some more points:

1. This line should be moved below the docstring
2. This still won't work
3. Same with this
4. If you're modifying this view straight from the stripe package, I'd suggest extending it, adding just this line and overwriting the urlconf entry for the view.

I changed "discount_code" to "coupon", I just didn't show that part of the code in my latest comment. I moved the docstring. I'm not sure what you mean in your fourth point. How do I extend? Do I just make a view in my own project that has the same title like I've done with the subscribe_form template?

yourviews.py

from djstripe.views import SubscribeFormView

class MySubscribeFormView(SubscribeFormView):
    form_valid_message = "Congratulations! You've successfully subscribed to Zana Edge. We'll be in touch shortly via email."

yoururls.py

from .views import MySubscribeFormView

url("^subscribe/$", MySubscribeFormView.as_view(), name="mysubscribe")

I'm not sure if setting the name to djstripe:subscribe and adding it above the djstripe url import will work. Give it a try though - if it doesn't work, you just need to change the action on the subscribe form here to {% url 'mysubscribe' %} either by overriding the entire template or setting it via javascript. If you've already customized that template (djstripe/subscribe_form.html) and haven't overridden it outside the package, I can help with that too.