07 August 2018
You can't predict by 100%, but you can limit your field only on characters that will appear in PO Number. For example, if your PO Number contains only numbers, you will select only digits. Or if your order number contains letters in prefix, you can add letters by 30% and digits by 70%. Or if you know that it starts with "7200" and will have 6 more digits then you can add regular expression that PO number will start with 7200 and can contain only numbers and length will be 10 digits.
These are only examples, there are many combinations. First investigate how PO number is generated, and then use the example from the link I provide you earlier.