Type the directives you want to add into the Custom Settings field.
To save the changes click the Update button.
Note: The changes will be applied to the currently selected website. The hostname of the currently selected website is shown both at the Site tab and at the top of the left pane. If you have several websites, make sure that the right website is selected. Otherwise, click on the Select Another Site link at the left pane, select the right website and be back to the website custom settings.