InDesign Keyboard Shortcut for Submenu Item

I am attempting to make a keyboard shortcut for a particular menu selection in InDesign CS5. Particularly, I want to select an Adobe PDF Preset

("PDF/X-1a:2001") from the submenu. However, when I try select this in Keyboard Shortcuts, all I get is "Define" .. which of couse will bring me to the presets window. I want to be able to select a preset and assign a keyboard shortcut (in this case F15).

Does anyone have suggestions on how this can be done? I tried various ways, but still can't select anything from that submenu.

Thanks.