Why I can't click this button?

Question

Hi... I want to click this "Finish" button with my Bot...





but I can't.. I want to click this button automatically when I click my button1 in C# project.

Html Codes:
  <br><br><input name="op" value="finish" type="hidden"><input name="XOOPS_TOKEN_REQUEST" id="XOOPS_TOKEN_REQUEST" value="1bf59f1d3289e154ee0274ae9f2a045b" type="hidden"><input value="Finish" type="submit"></form>

and I write:

  private void button1_Click(object sender, EventArgs e)
  {
  webBrowser1.Document.GetElementById("XOOPS_TOKEN_REQUEST").InvokeMember("click");
  }

but this code doesn't work... Help me please..

you can find this finish button when you fill register form in this page:
http://sfj.grigori.org/register.php

Cheers...