If it's 7.x custom client it shouldn't be too difficult, just think of the concept and try to apply it to your code:
Code:
private void updateClientList(object sender, RoutedEventArgs e)
{
foreach (Process p in Process.GetProcessesByName("Tibia"))
{
clientList.Items.Add(p.PID.ToString());
}
}
public void onUserSelectClient(object e, EventArgs a)
{
foreach (Process p in Process.GetProcessesByName("Tibia"))
{
if (p.PID.ToString() == clientList.selectedItem)
{
// Set your client to p
}
}
}
You might also like to identify the client by reading the name etc from within, that's where things get a little tricky and you'll need to know what version of Tibia, as well as to work out exactly where in memory the name is stored.