Append new fields based on drop down list

Hi all,
I'm writing a script appending new Standardize fields in all projects. Is there any way to let users choose the fields manually from a dropdown list to make the formula for these Standardize fields? 
 
E.g: In the picture attached, users need to choose the correct fields manually (Users or Users_2) to append createdBy field which is a standardize field. The name of standardize fields remain unchanged among projects.