go to the server behaviors panel (window -> Server Behavors) and double click the universal email server behavior.
Next to the subject, click the lightning bolt and select the text field from the form collection.
This will add code similar to this for the subject:
<?php echo((isset($_POST["textField"]))?$_POST["textField"]:"") ?>
you can type the static part of your subject line in front of that code:
Online Submission Form <?php echo((isset($_POST["textField"]))?$_POST["textField"]:"") ?>