HTTP POST with raw body - Flutter/Dart

I am getting this error

flutter: response body:{"Message":"The request entity's media type 'text/plain' is not supported for this resource."}

which I am getting in post whenever I am changing the body type to text instead of json. How can I solve this. here are the postman screenshots of both successful request and field request are given below

Here is the post method

Future<String> sendRequest() async {
      final jsonArray = json.encode(body);
      algo.Encrypted encrypted = Algorithm.encrypt(algorithmKey, jsonArray);
      final encryptedBosy = jsonEncode({'body': encrypted.base64});
      final response = await https.post(
        Uri.parse("$baseUrl/$paymentMethod"),
        headers: {
          "Key": key,
          "secretKey": secretKey,
        },
        body: encryptedBosy,
      );
      return response.body;
    }

Add the content type header to the request.

headers: <String, String>{
  'Content-Type': 'application/json; charset=UTF-8',
},