Hi Natalie,
You can set up validation by selecting the text box, right clicking, choosing properties, display, and conditional formatting. Add a condtion that matches a pattern and this patter should correspond to /d/d/d-/d/d/d where /d is a number. This will force people to enter the field in the format you require. In addition, I'd add placeholder text to the field that says "please enter in 000-0000 format" and maybe even a screen tip. You can acccess those guidance text options via the properties dialog.
Best of luck!