Add Option For Code To Request Source Column?

The below code uses data in column "A" (Const cl& = 1) to create a sheet for every unique value in column "A".

I would like for the code to request what column to use as source data. As an example when the code first runs a pop-up box would ask for a column letter to use as the source, after entering the letter (or corresponding column number) the code will execute.

Code:
Sub Add_sheets_from_A()
Range("XA1") = ActiveSheet.Name
ActiveSheet.Name = ("Add_Sheets")

[Code]....