title: TypeError: 'ImmutableMultiDict' objects are immutable python
body: <p>Hi I'm trying to add a new key value pair but I'm receiving an error of 
  <strong>TypeError: 'ImmutableMultiDict' objects are immutable</strong> the variable I'm trying to add a new key came from a <strong>request.form</strong> but I can't add a new key value. Any idea on how to achieve this?</p>
  
  <p>Here is my code on my controller</p>
  
  <pre><code>@benefits_api.route("/templates", methods=["POST"])
  def store():
  
      parameters = request.form
      response = BenefitTemplateService.create(parameters)
  
      return jsonify(response), response['code']
  </code></pre>
  
  <p>and my service is like this</p>
  
  <pre><code>class BenefitTemplateService(object):
  
      @staticmethod
      def create(params):
          # some validation here
  
          params['credit_behavior'] = "none"
          return params
  </code></pre>
  
  <p>But I'm getting an error on the assignment for credit_behavior below is the error message</p>
  
  <p><a href="https://i.stack.imgur.com/oa5jl.png" rel="noreferrer"><img src="https://i.stack.imgur.com/oa5jl.png" alt="enter image description here"></a></p>