Adding Dashes Automatically into Text Box

Hi,

I'm looking to add dashes automatically into a text box.

The text box is used for entering a number in this format: ###-###-###

How do I add the dashes automatically? I totally lost.

BTW, I'm coding in VBA if that makes a difference

Thanks!

EDIT:
So the user enters into textbox: 123456789
And it automatically becomes: 123-456-789
Thanks.