Not exactly a single key, but you can assign a "double tap" from a modifier key. For example: Option+Option
If you want a single key, you would need another application that can customize your key configuration from a lower level like Karabiner Element. I'm not using it since I didn't think of any tricks that I would use from it (yet), but I think this should make it possible for you to do what you want.
However, just my 2 cents, honestly I don't know why I would assign my Option key to limit it to only pop Alfred since, as much as I love Alfred, my Option key is pretty useful to insert symbols. So I would be more inclined to use the Double Tap instead of going the Karabiner route. But you can try both and see what you prefer. Or, as an idea, you could set only your right (or left) Option key to pop Alfred so you have your other Option key to work normally
