Outlook Field Valadation Formula Help

Can someone help me please, I need a validation formula that will require a sender of a form to enter 10 characters in a field.

The filed is called “File No” it’s on the default email form. I made it so it requires data with… Format([File No],”XXX###-###”) but it still allows users to only enter 3 letters to send and I need it to require all 10 charachers i.e. sdr234-547

Can anybody help PLEASE?