ASMX: Setting User/Password at run-time

Question

When I want to connect to my web service If I write it like this:

m_TransferServiceSoap.Credentials = System.Net.CredentialCache.DefaultNetworkCredentials;

it is working. but I need a Login Form. so users can enter their user/pass

so I am creating a NetworkCredential instance and setting its user/pass members from what I get from that login form and then instead of using the line above I am writing this one:

m_TransferServiceSoap.Credentials = userpass; // the instance above.

but in this way when I call a method of that webService, it gives me errors .

Is it the wrong way to set credentials?