Is there a way of sending form values without inputs

Let say that I have the form below:

<form action="sendMessage.php" method="post">
    <label>Subject</label><input type="text"></input>
    <label>Message</label><input type="text"></input>
    <input type="submit"></input>
</form>

How can I send an another value like topic(which is pre-defined) without creating an another element for that? Is it possible or do I have to make input element and style it with display: none;

The answer can contain html, javascript or jquery code.