Getting Error 4 when calling from C# HttpClient

I am hitting the batch command using C# HttpClient class and keep on getting error code 4 but absolutely the below format is fine.

I am also encoding and sending the payload like below

var content = new StringContent(HttpUtility.UrlEncode(mainParam), Encoding.UTF8, "application/x-www-form-urlencoded");

mainParam is shown below, can you tell me how to fix this issue please ?

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Number","v":""}},{"svc":"item/update_custom_field","params":{"itemId":16141282,"callMode":"create","id":0,"n":"Fuel Id","v":""}},{"svc":"item/update_custom_field","params":{"itemId":16141282,"callMode":"create","id":0,"n":"Install Date","v":"8/22/2017"}},{"svc":"item/update_custom_field","params":{"itemId":16141282,"callMode":"create","id":0,"n":"Install Technician Name","v":""}},{"svc":"item/update_custom_field","params":{"itemId":16141282,"callMode":"create","id":0,"n":"License Country","v":""}},{"svc":"item/update_custom_field","params":{"itemId":16141282,"callMode":"create","id":0,"n":"License Expiration Date","v":""}},{"svc":"item/update_custom_field","params":{"itemId":16141282,"callMode":"create","id":0,"n":"License State","v":"  "}},{"svc":"item/update_admin_field","params":{"itemId":16141282,"callMode":"create","id":0,"n":"Device 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