Capture a select value in a view of django

I do not know how to capture the value of a select from a form created from a ModelForm. in the template:

<div class="form-group">
  <label for="{{formulario.cod_sucursal.id_for_label}}"> 
{{formulario.cod_sucursal.label}}</label>
   <select name="{{formulario.cod_sucursal.id_for_label}}" id="{{formulario.cod_sucursal.id_for_label}}" class="custom-select form-control">
     <option value='0'>Selección de Sucursal</option>
       {% for opciones in formulario.cod_sucursal %}
         {{opciones}}
         {% endfor %}

   </select>
</div>

The form is cut off because it is longer.

What is rendered is:

<form action="/tablaventas/recibo" method="post">

  <input type='hidden' name='csrfmiddlewaretoken' value='0w3Y3WLmT6rbN488ohtG36LUMcS2t6XrhDu3X26V7fecg35kMkOzyv5kNXTRFu1Z' />

    <div class="form-group">
      <label for="encabezado-cod_sucursal">Cod sucursal</label>
        <select name="encabezado-cod_sucursal" id="id_encabezado-cod_sucursal" class="custom-select form-control">
          <option value='0'>Selección de Sucursal</option>

          <option value="600">Pilgrim</option>

          <option value="200">Mandiola</option>

          <option value="100">Fch</option>


        </select>
      </div>

the views.py for the function is:

def tablaventas_recibo(request):
if request.method=='POST':
    formu=VentasFormSet(request.POST, prefix='detalle') # formset 
    encabezado=tablaVCPFormulario(request.POST, prefix='encabezado') # Formulario
    print(encabezado.errors)
    if formu.is_valid() and encabezado.is_valid():
        codigo_sucursal=encabezado.cleaned_data

        co = formu.cleaned_data[1]
        print(codigo_sucursal)
        return HttpResponse('gracias')


return HttpResponse(encabezado.errors)

and the ModelForm is:

class tablaVCPFormulario(ModelForm):
    class Meta:
    model=VentasContrapuntoPilgrim
    fields='__all__'
    widgets = {
       'titulo': TextInput,
       'edicion': TextInput,
       'fecha_contable':SelectDateWidget(attrs={'class': 'custom-select'}),
       'cod_sucursal':SelectMultiple(choices=codigos_sucursal),
    }
    exclude = ('saldo','inicial','isbn',)
    prefix='encabezado'

the error output tells me I need to enter cod_sucursal , just that it is in the html select in the template, but I have no idea what I will need to be captured and sent in the post . Muchach @ s, of course, very grateful.