Add user's email when sending email in Django Rest Framework

Question

I've got this working serializer:

class ReportAnswerSerializer(serializers.ModelSerializer):
    complain_by = serializers.PrimaryKeyRelatedField(read_only=True, default=serializers.CurrentUserDefault())
    answer = serializers.PrimaryKeyRelatedField(read_only=True)

    class Meta:
        model = ModerateAnswer
        fields = ('answer', 'moderate_reason', 'moderate_comment', 'complain_by')
        extra_kwargs = {'moderate_reason': {'required': True}}

    def create(self, validated_data):
        instance = super(ReportAnswerSerializer, self).create(validated_data)
        send_mail(
            'Dziękujemy za zgłoszenie',
            'Zgłosiłeś odpowiedź: {}. Team Deor zajmie się teraz tą wiadomością'.format(instance.answer),
            '[email protected]',
            [[email protected]],
            fail_silently=False,
        )
        return instance

But i don't want to statically send email. Everytime a user reports an answer i want to send them a mail "Thanks for reporting the answer" But i dont know how to format the [[email protected]] dynamically. I am using a custom user model.

class CustomUser(AbstractUser):
    first_name = models.CharField(max_length=50, null=True)
    last_name = models.CharField(max_length=50, null=True)
    slug = models.SlugField(max_length=30, unique=True, null=True, blank=True, editable=False)
    email = models.EmailField(blank=False, unique=True)
    number_of_points = models.IntegerField(default=0)
    facebook_profile = models.CharField(max_length=128, null=True, blank=True)
    instagram_profile = models.CharField(max_length=128, null=True, blank=True)
    twitter_profile = models.CharField(max_length=128, null=True, blank=True)
    package = models.CharField(max_length=128, null=False, default='L')
    paid_till = models.DateField(null=True, blank=True)
    package_downgraded = models.CharField(max_length=128, null=True, blank=True)
    sex = models.CharField(max_length=128, null=True, blank=True)
    location = models.ForeignKey(City, on_delete=models.CASCADE, null=True, blank=True)
    badges = models.ManyToManyField('users.Badge', blank=True)

Do you have any idea how to do it?