forms.py

def validate_user(user_id):
    try:
        User.objects.get(pk=user_id)
    except User.DoesNotExist:
        raise forms.ValidationError('Usuário inválido')
          
      
class CriarReserva(forms.ModelForm):
    usuario_id = forms.IntegerField(validators=[validate_user])
    
    class Meta:
        model = Reserva
        fields = ('apartamento', 'data_entrada','data_saida','qtdHospedes')
    
    def save(self, **kwargs):
        self.cleaned_data['usuario'] = \ 
            User.objects.get(pk=self.cleaned_data.pop('usuario_id'))
        return super().save(**kwargs)