I want to add a field in the User Preference, Custom Preference. I see that the fields look like custscript_9997_file_gen_saved_search. When can I go to add custscript ?

This would be a script parameter

Any documentation in Netsuite ? I don't see how to access the script for User Preference / Custom Reference

In order to use the field, it requires a script to be written. This would be on the parameter page of the script setup. If that's not enough to point you in the right direction, I'm going to be very blunt and tell you that you've no business trying to use one