In JavaScript, you can find a particular FreeTextBox control using the following syntax:

 <script type="text/javascript">
   ftb = FTB_API['FreeTextBox1'];
   ftb.InsertHtml("some <b>text</b>");
 </script>

In this example 'FreeTextBox1' is the ClientID of the FreeTextBox. The ClientID is generated by the .NET framework and can be accessed by getting ClientID.