Unable to set json string error while making post request to third party api.

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JSON

Request of json is in below format which is stored in string DATA. (below is just part of json)
...............
..............

<PortfolioList>
                                            <Portfolio>
                                                <PortfolioGroup>1<\/PortfolioGroup>
                                                <WalletDeposit>0.0<\/WalletDeposit>
                                                <MediaList>
                                                    <Media>
                                                        <MediaStatus>0<\/MediaStatus>
                                                        <MediaType>0<\/MediaType>
                                                        <MediaSerial>1<\/MediaSerial>
                                                        <EncodeDateTime>2016-05-23T09:05:14.013+0200<\/EncodeDateTime>
                                                        <EncodeFiscalDate>2016-05-23<\/EncodeFiscalDate>
                                                        <ExclusiveUse>true<\/ExclusiveUse>
                                                        <MediaCodeList>
                                                            <MediaCode>
                                                                <MediaCode>1M6BYWR7XB5BXE<\/MediaCode>
                                                                <MediaCodeType>0<\/MediaCodeType>
                                                            <\/MediaCode>
                                                        <\/MediaCodeList>
                                                    <\/Media>
                                                <\/MediaList>
                                            <\/Portfolio>
                                        <\/PortfolioList>

..............
.......
..... and so on...

The problem is I am not able to read this specific format like "<portfoliolist> <\/PortfolioList> " which is coming in json request.

What I have tried:

As described in problem i have stored complete format in string DATA.

string DATA = @"{Header: {WorkstationId: " + WorkstationId +
@"}, Request: {......

System.Net.Http.HttpClient client = new System.Net.Http.HttpClient();
client.BaseAddress = new System.Uri(SnAPIUrl);
client.DefaultRequestHeaders.Accept.Add(new System.Net.Http.Headers.MediaTypeWithQualityHeaderValue("application/json"));

System.Net.Http.HttpContent content1 = new StringContent(DATA, UTF8Encoding.UTF8, "application/json");
HttpResponseMessage messge = client.PostAsync(SnAPIUrl, content1).Result;

if (messge.IsSuccessStatusCode)
{
string result = messge.Content.ReadAsStringAsync().Result;
// Error is coming here in result. (complete format is coming in varibale with error message : Unable to set json string)
//return result;
}