I am attempting to set up a custom error page for formmail when required fields are not filled in. I am using the recommendation given for the "missing_fields_redirect" field description. I'm able to get it to the error page, however, what I would like to have is a message stating which required fields were missing just like the default error page does.
I presume that one has to set up a dynamic page and capture values to determine what to display. I can do this but I don't know what to look for.