FCKEditor Help

I use Fckeditor to enter text in one of my site forms. All I want is that the user should atleast enter 100 characters or else a popup should come up.
Can anyone help me with this

 

<form name="name-of-form" method="get" action="http://www.yourdomain.com" onsubmit="showpopup()">
<input type="text" name="textbox-name">
<input type="submit" value="Submit form">
</form>

Code (markup):

function showpopup() {
  if (name-of-form.textbox-name.value.length < 100) {
    alert('You entered less then 100 characters!!!');
    return('false');
  }
  else {
    return('true');
  }
}

Code (markup):

 

Its a late reply but thank you