Create list item in SharePoint list using REST API

 HttpWebRequest contextinfoRequest =

    (HttpWebRequest)HttpWebRequest.Create(sharepointUrl.ToString() + "/_api/contextinfo");

contextinfoRequest.Method = "POST";

contextinfoRequest.ContentType = "text/xml;charset=utf-8";

contextinfoRequest.ContentLength = 0;

contextinfoRequest.Headers.Add("Authorization", "Bearer " + accessToken);

HttpWebResponse contextinfoResponse = (HttpWebResponse)contextinfoRequest.GetResponse();

StreamReader contextinfoReader = new StreamReader(contextinfoResponse.GetResponseStream(), System.Text.Encoding.UTF8);

var formDigestXML = new XmlDocument();

formDigestXML.LoadXml(contextinfoReader.ReadToEnd());

var formDigestNode = formDigestXML.SelectSingleNode("//d:FormDigestValue", xmlnspm);

string formDigest = formDigestNode.InnerXml;

//Execute a REST request to get the list name and the entity type name for the list.

HttpWebRequest listRequest =

    (HttpWebRequest)HttpWebRequest.Create(sharepointUrl.ToString() + "/_api/Web/lists(guid'" + listId + "')");

listRequest.Method = "GET";

listRequest.Accept = "application/atom+xml";

listRequest.ContentType = "application/atom+xml;type=entry";

listRequest.Headers.Add("Authorization", "Bearer " + accessToken);

HttpWebResponse listResponse = (HttpWebResponse)listRequest.GetResponse();

StreamReader listReader = new StreamReader(listResponse.GetResponseStream());

var listXml = new XmlDocument();

listXml.LoadXml(listReader.ReadToEnd());

//The entity type name is the required type when you construct a request to add a list item.

var entityTypeNode = listXml.SelectSingleNode("//atom:entry/atom:content/m:properties/d:ListItemEntityTypeFullName", xmlnspm);

var listNameNode = listXml.SelectSingleNode("//atom:entry/atom:content/m:properties/d:Title", xmlnspm);

string entityTypeName = entityTypeNode.InnerXml;

string listName = listNameNode.InnerXml;

//Execute a REST request to add an item to the list.

string itemPostBody = "{'__metadata':{'type':'" + entityTypeName + "'}, 'Title':'" + newItemName + "'}}";

Byte[] itemPostData = System.Text.Encoding.ASCII.GetBytes(itemPostBody);

HttpWebRequest itemRequest =

    (HttpWebRequest)HttpWebRequest.Create(sharepointUrl.ToString() + "/_api/Web/lists(guid'" + listId + "')/Items");

itemRequest.Method = "POST";

itemRequest.ContentLength = itemPostBody.Length;

itemRequest.ContentType = "application/json;odata=verbose";

itemRequest.Accept = "application/json;odata=verbose";

itemRequest.Headers.Add("Authorization", "Bearer " + accessToken);

itemRequest.Headers.Add("X-RequestDigest", formDigest);

Stream itemRequestStream = itemRequest.GetRequestStream();

itemRequestStream.Write(itemPostData, 0, itemPostData.Length);

itemRequestStream.Close();

HttpWebResponse itemResponse = (HttpWebResponse)itemRequest.GetResponse();

RetrieveListItems(accessToken, listId);