Input Mask for Field

Summary

How do I formulate a numeric Input Mask for a field

Content

Hello,

We want to make a certain field on a form only allow 6 numbers as the input on the candidate facing side. I know that I can go to the form and click on the field and scroll to the bottom and I see the "Input Mask" line. My question is what do I put in there? 6 X's? Or is there another way that I need to formulate this input mask? Any thoughts?

Thanks,

Stephanie