To add the fields first_name and last_name from the default Django User model provide your own formclass:
Prepend the two fields to Meta.fields of the default RegistrationForm:
from django_registration.forms import RegistrationForm
class RegistrationWithNameForm(RegistrationForm):
    class Meta(RegistrationForm.Meta):
        fields = ["first_name", "last_name"] + RegistrationForm.Meta.fields
Override the default RegistrationView by adding a path to urls.py:
from django_registration.backends.activation.views import RegistrationView
from yourapp.forms import RegistrationWithNameForm
path('accounts/register/',
  RegistrationView.as_view(form_class=RegistrationWithNameForm),
  name='django_registration_register',
),
path("accounts/", include("django_registration.backends.activation.urls")),
Testcase:
from django.contrib.auth import get_user_model
from django.test import TestCase
from django.urls import reverse
class RegistrationTestCase(TestCase):
    registration_url = reverse("django_registration_register")
    test_username = "testuser"
    post_data = {
        "first_name": "TestFirstName",
        "last_name": "TestLastName",
        "username": test_username,
        "email": "testuser@example.com",
        "password1": "mypass",
        "password2": "mypass"
    }
    def test_register(self):
        response = self.client.post(self.registration_url, self.post_data)
        self.assertRedirects(response, reverse("django_registration_complete"))
        user = get_user_model().objects.get(username=self.test_username)
        # Assert all fields are present on the newly registered user
        for field in ["username", "first_name", "last_name", "email"]:
            self.assertEqual(self.post_data[field], getattr(user, field))