I have the following two model classes in django rest framework

class Customer(models.Model):
    GENDER_CHOICES = [
    ('Male', 'Male'),
    ('Female', 'Female'),
   ] 
    customer_id = models.AutoField(primary_key=True)
    firstname = models.CharField(max_length=20)
    lastname = models.CharField(max_length=30) 
    gender = models.CharField(max_length=6, choices=GENDER_CHOICES)
    date_of_birth = models.DateField()
    nationality = models.CharField(max_length=50, null=False)
    id_card_number = models.CharField(max_length=20, unique=True)
    passport_number = models.CharField(max_length=20, unique=True)
    address = models.CharField(max_length=100)
    phone_number = models.CharField(max_length=20)
    email = models.EmailField(max_length=254,null=True, blank=True, unique=True)
    
    def __str__(self):
        return f"{self.firstname} {self.lastname}"
    
    
class Contract(models.Model):    
    INSURANCE_TYPE_CHOICES = [
        ('Health', 'Health Insurance'),
        ('Life', 'Life Insurance'),
        ('Auto', 'Auto Insurance'),
        ('Home', 'Home Insurance'),
    ]
    
    contract_number = models.AutoField(primary_key=True)
    start_date = models.DateField()
    end_date = models.DateField()
    insurance_type = models.CharField(max_length=15, choices=INSURANCE_TYPE_CHOICES)    
    insurance_holder = models.ForeignKey(Customer, on_delete=models.CASCADE, related_name="contracts")
    active = models.BooleanField(default=True)
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)
     
    
    def __str__(self):
        return f"{self.contract_number} {self.insurance_type}"

and the following two serializers

class ContractSerializer(serializers.ModelSerializer):
    
    insurance_holder = serializers.StringRelatedField(read_only=True)
   
    class Meta:
        model = Contract
        exclude = ("updated_at",) 
    

class CustomerSerializer(serializers.ModelSerializer):
    
    contracts = ContractSerializer(many=True, read_only=True)   
    
    class Meta:
        model = Customer
        fields = "__all__"

I have also created my url endpoints and my views, i have tested my api through Postman and the django-rest built in
browasable API and everything is fine but the problem i have is the following.

The insurance holder is a foreign key of my customers model so when i do get requests in contracts endpoint i get the Id
instead of the full name so I set the insurance holder in the serializer as StringRelatedField to get the fullname, however this is read only field so i am not able to post requests in contracts with the fullname. How can i make post requests with the name but retain the foreign key constraint of the models.

By the way i am new in Django and Django Rest Framework and novice just in general.

I already read django rest documentation about related fields and the following stack overflow post How can I show the StringRelatedField instead of the Primary Key while still being able to write-to that field using Django Rest Framework? but i still don’t understand how to solve the problem..