Rainloop is preconfigured to work with i-MSCP. You should not change any settings. If you want really do, you can edit the /var/www/imscp/gui/public/tools/rainloop/data/_data_11c052c218cd2a2febbfb268624efdc1/_default_/configs/application.ini file and set the allow_admin_panel option to On. Once done, you can connect with the following credential:
username: admin password: 12345
Be aware that no support will be provided if you broke something.
