Ron,

you need to check first, how the other app accepts login credentials in the URL and how parameters need to be named and which security option you have. simpliest will be to pass a hashed password, a more secure solution first does a handshake between servers, where DNN server gets a token from the other application, valid for e.g. 1 minute and a single login, which is passed subsequently in the URL. In this situation, you might consider not to pass the DotNetNuke password but instead a hashed fixed password, so noone has the chance to gain user credentials for your DNN site by capturing the login URL.