Bikemonger says..... Sure it's only a loop of cord, but it's *nice* cord, and it's just right length, properly heat sealed and tied with a secure knot. Worth a few extra quid to save the faff of making one yourself. 

TowWhee says....  The Fast Stem Hook is an instant connection/disconnection accessory for the TowWhee Adventure Tow Bungee. The 2nd rider can easily make their own connections in an instant. A Fast Stem Hook is the fastest TowWhee connection method to the 2nd rider. It is an 8” loop of high end cord that holds its shape well. 

Loop the Fast Stem Hook through the TowWhee loop, then through itself. Pull tight as an extension of the TowWhee itself. Loop the Stem Hook over the handlebars, back under the stem.  The shape of the material will retain its shape and hold well in a variety of terrain.