How can I add a new directive using WHM advanced exim configuration editor?

NEVERMIND: I finally found the button to add new directives.

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In WHM --> Exim Configuration Manager --> Advanced Editor, I read this included in the top of the page:

To use, add new directives or modify current directives within the provided text boxes.

Okay, so I need to add delay_warning = 0h

I see that I can pull down any of the other configuration menus, e.g. the one that is currently there for auto_thaw, and I can select delay_warning, and then add the needed parameter. But the problem is, this removed auto_thaw from the list.

So how exactly can I ADD a new directive?

Thanks very much.