You have to prevent the KeyPressed event from being generated, that’s the one that beeps. That requires setting the SuppressKeyPress
property to true. Make that look similar to:
if ((e.KeyCode == Keys.Enter) || (e.KeyCode == Keys.Tab))
{
Parent.SelectNextControl(textBox_Zakljucak, true, true, true, true);
e.Handled = e.SuppressKeyPress = true;
}