Multiple Form Validation With One Function

I have been using this simple js function to validate a field in a form is filled in.

<script language="JavaScript">
<!--
function SubmitForm()
{
var form = document.formname
if (form.client_id.value.length == 0)
{
alert('Please make a selection from the list');
form.client_id.focus();
}
else
{
form.submit();
}
}

//-->
</script>

Now I have a page with multiple forms and I would like to use the same script to validate depending on which form is submitted. I thought I should be able to do something like this.... but know very little javascript...any suggestions? Code: