Why is "On Scan" unavailable in the script activation options?

I am attempting to create a Scan Parsing script similar to the one described in the following article:

Scripting: Parse Scan Data

The issue I'm having is selecting the option for script activation based on scanner input. No matter what I try, the option is unavailable:

I guess I could just add the script as Text and then manually add the activation line myself. Would that work?

     Activate( On_Input, sScan, nParse )