Implementing active authentication using ADFS

The reason those articles suggest you redirect (using WS-Federation protocol) to the ADFS login page is because it allows you to set up federation to other identity providers (allow an external company' employees to use their own credentials to log in to your application).

What you want can be done using the WS-Trust protocol, but you'll give up (or have to implement yourself) the possibility to federate.

ADFS exposes endpoints like /adfs/services/trust/13/usernamemixed that you can talk to to get a security token. Something like below should get you going.

public class UserNameWSTrustBinding : WS2007HttpBinding
{
    public UserNameWSTrustBinding()
    {
        Security.Mode = SecurityMode.TransportWithMessageCredential;
        Security.Message.EstablishSecurityContext = false;
        Security.Message.ClientCredentialType = MessageCredentialType.UserName;
    }
}

private static SecurityToken GetSamlToken(string username, string password)
{
    var factory = new WSTrustChannelFactory(new UserNameWSTrustBinding(), "https://yourdomain.com/adfs/services/trust/13/UsernameMixed")
        {
            TrustVersion = TrustVersion.WSTrust13
        };

    factory.Credentials.UserName.UserName = username;
    factory.Credentials.UserName.Password = password;

    var rst = new RequestSecurityToken
    {
        RequestType = RequestTypes.Issue,
        AppliesTo = new EndpointReference("https://yourdomain.com/yourservice"),
        KeyType = KeyTypes.Bearer
    };

    var channel = factory.CreateChannel();

    return channel.Issue(rst);
}