daydie 12 Junior Poster in Training

When i make a login for vb forum, the post string i get back on real login is :

vb_login_username=the_legend&vb_login_password=&vb_login_password_hint=Password&s=&securitytoken=1329916133-7066266c7073fe1bf6b02ad340d25fa9a55513ca&do=login&vb_login_md5password=a033848a48e9e12b9cef05d31e7d991b&vb_login_md5password_utf=a033848a48e9e12b9cef05d31e7d991b

But the real string to login successfully by application POST is the following

raw_login_password=" & TextBox2.Text & "&do=login&url=%2Fforum%2Fusercp.php&vb_login_md5password=" & MD5(TextBox2.Text) & "&vb_login_md5password_utf=" & MD5(TextBox2.Text) & "&s=&securitytoken=guest&vb_login_username=" & TextBox1.Text & "&vb_login_password=&cookieuser=1

How come? The real POST will not work on application POST but the second one will. It did not show that raw_login_password was needed on original POST via webbrowser (From LiveHTTPHeaders)

Just to let you know i found the second string on the web, how come i couldn't see it? and how would i be able to find this hidden field in future reference?