This question already has an answer here:

I need to input a csv as parameter 1. Parameter 2 should be a dropdown displaying the various fields (the headerline) where I can select a column to use for geoprocessing.

  • Param1: c:\my.csv
  • Param2: (dropdown) select header

merged by PolyGeo Nov 19 '15 at 21:46

This question was merged with ArcGIS Python Script Tool Field Name Drop Down? because it is an exact duplicate of that question.