Call FireOnChange event MS CRM 4.0 via webBrowser Control in C#

Hi,

i would like to fire the event FireOnChange event in a webBrowser Control for an Outlook Plugin, but the event won't be fired. This function should be executed when i leave the name field for a duplicate function check in JS.

 webBrowser1.Document.InvokeScript("crmForm.all.lastname.FireOnChange();");

plz help thx

Andreas