Pad leading zero to numbers - detailed instructions needed

I'm brand new to Acrobat with no knowledge of postcript.

I'm creating a form from an existing document, and finding it rather easy for almost all the fields except one which I have formatted it to accept a numeric input with no decimals, and validation of the value between 1 and 99999. What I really want to have leading zero's padding to the front/left if the input is less than 5 digits (so that it will also have 5 digits on the form). For example, if user enters 123, I want it to become 00123.

It looks like I probably need to choose a "custom" format but I don't know what to put in "Custom Format Script" and/or "Custom Keystoke Script". 

Hope someone can help!