How to make the button perform the actions that are written in the handler?

Question

Django

I want to add this anime to the manytomany field of the model when clicking on the button located on the anime page. I can't understand what I'm doing wrong because I'm doing it for the first time, maybe I didn't write the handler or the button or the form correctly, please help.

Model:

class Profile(models.Model):
    user = models.OneToOneField(User, on_delete=models.CASCADE)
    bio = models.TextField('Описание', max_length=500, blank=True)
    avatar_image = models.ImageField('Аватар', upload_to='avatar/', blank=True)
    shapka_image = models.ImageField('Шапка', upload_to='shapka/', blank=True)
    watching = models.ManyToManyField(Anime, verbose_name='Смортю', blank=True, related_name='users_watching')
    willwatch = models.ManyToManyField(Anime, verbose_name='Буду смотреть', blank=True, related_name='users_willwatch')
    viewed = models.ManyToManyField(Anime, verbose_name='Просмотрено', blank=True, related_name='users_viewed')
    thrown = models.ManyToManyField(Anime, verbose_name='Брошено', blank=True, related_name='users_thrown')
    favorite = models.ManyToManyField(Anime, verbose_name='Любимые', blank=True, related_name='users_favoritepro')

Form:

class ProfileWatchingForm(forms.ModelForm):
    class Meta:
        model = Profile
        fields = ('watching',)

HTML:

<form action="." method="post">
      {% csrf_token %}
      <input type="submit" value="Watching anime" name="add_anime">
    </form>

views:

def watching_anime(request):
    if request.POST:
        if 'add_anime' in request.POST:
            prof = Profile.objects.get(pk=request.user.id)
            url = request.method['url']
            anime = Anime.objects.get(url=url)
            watch_form = ProfileWatchingForm(prof.watching.add(anime))
            watch_form.save()
    else:
        watch_form = ProfileWatchingForm()
    return render(request, 'anime/anime_detail.html', {'prof': prof, 'watch_form': watch_form})