How calling VB function from java script in gizmox webgui application ?

Hi everyone,
In my Gizmox Webgui application (vb code) I have: "Form1" containing an "HtmlBox1" control In the form I have the following function:

"<PermissionSet(SecurityAction.Demand, Name:="FullTrust")> _
<System.Runtime.InteropServices.ComVisibleAttribute(True)> _
Public Sub VbHelloWorld()
MsgBox("Hello Ridha Ammar")
End Sub"

In order to call this function from "HtmlBox1.html" I added to the html code of the "HtmlBox1" control the following script:

"function showVbFunction (){
window.external.VbHelloWorld ();
}"

and the following link:

"<a href="#"> Say Hello World From VB </a>"

When debugging and clicking the link, I get the following error:
"window.external.VbHelloWorld is not a function"

The script cannot access the function.
Is there anyone who can help me?