Requirement to Make the Secondary Quantity Field NULL and Mandatory using Page Customization

We have requirement to make the Secondary Quantity as 'NULL' and 'Mandatory' using Expression Builder through page customization for Dual UOM Items in all the Inventory Transactions. While doing it we encountered an issue, where we were able to Make the Secondary Quantity as 'NULL', but we can't restrict it based on the User Selection of a particular line e.g. Let's say there are multiple lines for receiving, so upon creating a receipt if the user clicks on the 2nd line, then the Secondary Quantity is auto-populated based on the UOM Conversion setup on the 2nd line and we can't