How can I add a Drawing Markup/Annotation to my stamp while it's being stamped?

I have created Dynamic Stamp and works perfect and its located in my user profile:

C:\Users\XXXXX\AppData\Roaming\Adobe\Acrobat\10.0\Stamps\symbols.php

And this works just fine...

Now I want to add a line to the stamp.

I want the user to introduce the length of the line using a response box (app.response)

For some reason when I try to add the line using:

var new_line = this.addAnnot({

  type:"Line",

  page:6,

  points:[[36,373],[613,373]],

  width:2, 

  style:"S ",

  strokeColor:color.black,

  arrowEnd:"None",

  arrowBegin:"None"

});

After this, the variable "new_line" is NULL, can somebody help me here?

So I cant add the line using JavaScript

Ironically when I try to create a field (text box) using JavaScript it works perfectly ....

var oField=this.addField({

cName: "info",

cFieldType: "text",

nPageNum: 6,

oCoords: [ 20, 20, 1, 1 ]

});

oField.value = "Hello World !!!";

I am able to insert the line manually when I open the stamp file in Acrobat and add the line under Comment->Drawing Markups->Draw Line. However I want the length of the line to be inserted by the user when the user applies the stamp... Can anybody help me here?