In script version >=v1.5.9 this feature should work by default or after entering your url in:
In script version <1.5.9:
Same thing in: application\views\admin\components\page_head_main.php same thing.
In application\config\config.php , enter base_url with http:// prefix
If you want auto redirect this can be done by server configuration, please ask hosting provider or find option in your hosting admin interface.