def save(self, commit=True):
    user = super(RegisterForm, self).save(commit=False)
first_name, last_name = self.cleaned_data['fullname'].split()
user.first_name = first_name
user.last_name = last_name
user.email = self.cleaned_data['email']
if commit:
    user.save()
return user
Django create custom UserCreationForm