Create alert message popup triggered by a invoice item level detail UDF value

I'm struggling to figure out how to create an alert message to warn users before taking action. Can someone provide me step-by-step instruction on how to do this? Here is what I'm looking to do:

Within A/R Invoice, I want to create a message alert popup that triggers when a user clicks the update button to save the invoice if a specific UDF on the invoice item level detail UDF has a non null value.