add dynamically requiered field validator?

hi any help how can i add a dynamically required field validator for my dymaically created text box? heres my code can anyone help me out to fix my problem.

<script type = "text/javascript">
var counter = 0;
function AddFileUpload() {
var div = document.createElement('DIV');
div.innerHTML = 'Description' + '&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;' +
'File' + '<br />' +
GetDynamicTextBox("") + '&nbsp; &nbsp;' +
'<input id="file' + counter + '" name = "file' + counter +
'" type="file" />' + Go to the complete details ...