Example: create a List Item

Example.CreateItem.cs

using (var client = new SPHttpClient(webUri, userName, password))
{
    var listTitle = "Tasks";
    var itemPayload = new {  __metadata = new { type = "SP.Data.TasksListItem" }, Title =  "Approval Task"};
    var endpointUrl = string.Format("{0}/_api/web/lists/getbytitle('{1}')/items",webUri,listTitle);
    var data = client.ExecuteJson(endpointUrl,HttpMethod.Post,itemPayload);
    Console.WriteLine("Task item '{0}' has been created",data["Title"]);
}