How to add leading symbol in a numeric field?

Put the following in the custom format script of the field.

The string starts with 10 asterisks then the number entered into the field gets commas inserted to separate the thousands and gets added to the 10 asterisks then we just grab the last 10 characters from the string and you get your padded value.

event.value = String("**********" + event.target.value.toString().replace(/\B(?=(\d{3})+(?!\d))/g, ",")).slice(-10);