without trying your code, I have another approach to suggest as an
alternative to the KeyDown event, which is to put  a CommandButton on the
form, and set its "Default" property to True (so it gets "Clicked" when the
user presses Enter), and on its Click event  add the TextBox contents  to the
ListBox and then set the focus  back to the TextBox. I THINK this would work.
As long as you don't mind having this new CommandButton there, and/or don't
already have a nother Default CommandButton...