SQL Developer: Tools -> Preferences -> Database

Filename for connection startup script

The Filename for connection startup script value allows to specify the path of an SQL script that is executed when SQL Developer opens a new database connection.

As per that Jeff Smith, it might contain at least set serveroutput on to enable dbms_output by default.

This value is stored in systemV.W.X.Y.Z/o.sqldeveloper/product-preferences.xml.

Compare with

The option Select default path to look for scripts, found under Tools -> Preferences -> Database -> Worksheet
$SQLPATH/login.sql

On the command line, the location of the startup script can be queried or set using the SQL Developer PowerShell module, functions get-SQLDeveloperStartupScript and set-SQLDeveloperStartupScript P:\ath\to\xyz.

Example script

set   serveroutput on   format wrapped
set   pages        5000
alter session set  plsql_warnings='enable:all';