How to set default value to list onformpopulate


I've list field on some of the document classifications which is mandatory field, I have to set default value, it is getting set on that field but while saving the document, getting error "Please provide value for mandatory property".

Below is code I written on "onformpopulate" event:

var documentItem = document.thisItem;

var type = documentItem.getProperty("classification");

if(type === "ABC" || type === "PQR")
     document.getElementById("MainDataForm").sh_number.value = "100";

How to handle this?