title: Fill value on hidden django input field on ModelForm
body: <p>I have a model form and I'm manually entering the fields into my template.</p>
  
  <p>Model form:</p>
  
  <pre><code>class ProductEntryForm(forms.ModelForm):
  
      class Meta:
          model = Product
          fields = [
              'image', 'product_name', 'price', 'options',
          ]
  </code></pre>
  
  <p>Manually rendering fields in Template:</p>
  
  <pre><code>&lt;form method="POST" enctype="multipart/form-data"&gt;
     &lt;h2&gt;New Product&lt;/h2&gt;
     {% csrf_token %}
  
     {{ form.non_field_errors }}
     &lt;p&gt;
        &lt;label for=""&gt;Image&lt;/label&gt;
        {{ form.image.errors }}
        {{ form.image }}
     &lt;/p&gt;
     &lt;p&gt;
         &lt;label for=""&gt;Product name&lt;/label&gt;
         {{ form.product_name.errors }}
         {{ form.product_name }}
     &lt;/p&gt;
     &lt;p&gt;
         &lt;label for=""&gt;Description&lt;/label&gt;
         {{ form.description.errors }}
         {{ form.description }}
     &lt;/p&gt;
     ...
  </code></pre>
  
  <p>The <code>options</code> field of the form is hidden in the template because the user doesn't fill this field directly. They fill it from a different input field.</p>
  
  <pre><code>&lt;form method="POST" enctype="multipart/form-data"&gt;
     ...
     &lt;p style="display: none"&gt;
         {{ form.options }}
     &lt;/p&gt;
  
     &lt;input type="text" name="visible_option_input" :value="[[make_this_value_of_{{form.options}}_when_submitted]]"&gt;
     ...
  &lt;/form&gt;
  </code></pre>
  
  <p>I want the value of <code>"visible_option_input"</code> to be the value of <code>{{ form.options }}</code> when submitted</p>