string message = "'Title': 'NewTitle2 updated'"
string json = "{ '__metadata': { 'type': 'SP.Data.MyTasksListItem' }," + message + "}";

function addItem(userId) {
    var due = new Date();
    due.setDate(due.getDate() + 7);

    var call = jQuery.ajax({
        url: _spPageContextInfo.webAbsoluteUrl + "/_api/Web/Lists/getByTitle('Tasks')/Items",
        type: "POST",
        data: JSON.stringify({
            "__metadata": { type: "SP.Data.TasksListItem" },
            Title: "Sample Task",
            AssignedToId: userId,
            DueDate: due
        }),
        headers: {
            Accept: "application/json;odata=verbose",
            "Content-Type": "application/json;odata=verbose",
            "X-RequestDigest": jQuery("#__REQUESTDIGEST").val()
        }
    });
    call.done(function (data, textStatus, jqXHR) {
        var div = jQuery("#message");
        div.text("Item added");
    });
    call.fail(function (jqXHR, textStatus, errorThrown) {
        failHandler(jqXHR, textStatus, errorThrown);
    });
}