Serialize a Dictionary as JSON object using DataContractJsonSerializer

I have an [DataContract] object that has some properties and is serialized to JSON using DataContractJsonSerializer.

One of the properties is of type Dictionary<string, string> and when the serialization happens it produces the following JSON schema.

"extra_data": [
  {
    "Key": "aKey",
    "Value": "aValue"
  }
]

Now I need the JSON schema to be like this

"extra_data": { 
        "aKey": "aValue"
}

You can never of course know before what the values are, it is a Dictionary that the user will set keys and values.

I'm thinking if this can happen using anonymous types, or is there a configuration I can take to accomplish my goal?

Thank you.