Lightroom Classic, Preferences not clear - How do I add items to 'external editing'?

I have Topaz software DeNoise and Sharpen but want to add Gigapixel to the list so I can get to it easily from within Lightroom while editing with the menu 'edit in...' but frustration trying to add Gigapixel to the list of external editors as Lightroom seems to not want to let me add another external editor. Goin in circles with this and don't want to cause damage because I don't know how to do this. What are the steps to adding an external editor to the list I get when option clicking on an image from within Lightroom? 

TIA,

Ken

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