Customizing Document Library Alert Mail Subject Line

Hi

I have a SharePoint 2013 Document Library. I have setup an alert whenever a new document gets added to the library.

The default alert mail subject line is of the format "<DocumentName> has been added" -

What I need is the subject line to be based on a custom column field (PONumber)-

So, something like - "PONumber 4205056558 has been added".

What would be the easiest way to approach this?

Thanks Arindam