You are attempting to retrieve the window handle of the Exchange login dialog of the MAPI application (probably outlook), FROM a rundll32.exe process. This can be done from .Net using the SendKeys class, but this is not completely reliable:
Alternatively you could establish the NETBIOS session in advance using NetUseAdd to establish a session. If this is successful, you will not need to enter the username and password. I don't know if this works with Exchange, but it does work with some other applications which use integrated authentication: