here is a good starter for using the EHLLAPI DLL that comes with Client
Access from .NET:
http://www.codeproject.com/csharp/ALL_EHLLAPI.asp

using the wrapper classes from CodeProject I wrote the following demo
code and it work first time!

private void button1_Click(object sender, EventArgs e)
{
UInt32 rc;
string dsplyBuf = "";
EhllapiWrapper ew = new EhllapiWrapper();
rc = ew.Connect( "A" ) ;
rc = ew.SendStr("sndmsg@4"); // sndmsg - f4
ew.ReadScreen(1, 256, out dsplyBuf);
textBox1.Text += dsplyBuf + Environment.NewLine;
rc = ew.Disconnect("A");
}

-Steve