sharepoint-csom-add-item.cs

// creates a new SharePoint context 
// passing the parameters
ClientContext clientContext = new ClientContext(siteUrl);
clientContext.Credentials = new NetworkCredential("wiliammbr", "password", "domain");

// selects the list by title
SP.List list = clientContext.Web.Lists.GetByTitle(listTitle);

// creates a new object to include the new list item
// this object helps you on defining which target folder the list item 
// is going to be saved and other settings
SP.ListItemCreationInformation itemCreateInfo = new SP.ListItemCreationInformation();

// adds the new item to the list with ListItemCreationInformation
SP.ListItem listItem = list.AddItem(itemCreateInfo);

// fill the fields with the required information
listItem["Title"] = "New Item!";
listItem["OtherField"] = "Some important information.";
listItem.Update();

// executes the creation of the new list item on SharePoint
clientContext.ExecuteQuery();