What I have done is
I have declared the name in two places

1) In html form textarea as ta_body_text
2) In the webform file I have delcared it once at the top
   public System.Web.UI.HtmlControls.HtmlTextArea ta_body_text;

3) Then the actual code executes in the onload section
     ta_body_text.Value = "test"

I was expecting test to be displayed in the box. Instead I get the error

"Object reference not set to an instance of an object"

samples would be appriciated
Thanks