Hi Janet - Since the Slug and Lettuce was the only thing that came up when I searched, my guess is that they invented it.
Their website just list the ingredients, which are:
Garden Fusion
Cucumber, fresh lime juice, sugar syrup and mint are muddled, topped with Red Bull and served long over crushed ice.
You could just try various amounts of each - muddle them, top them with Red Bull and give them a try until you duplicate it.
I am sure you already know that sugar syrup is just equal amounts of sugar and water heated until the sugar melts.
BBWCHATT
The old lady in Chattanooga, Tennessee, USA