When I developed this app (in C# Visual Studio 2008) I asked the same question (actually managed to find an answer on the MS forum, for which I deserve a prize of some sort)
You can use the global key hook up method mentioned over here: https://docs.microsoft.com/en-us/archive/blogs/vsod/using-shortcut-keys-to-call-a-function-in-an-office-add-in