With the installed position of the Slingshot battery being inaccessible without removing a panel, there is an alternative method we've used with success.
The alternator's positive post is readily accessible under the hood. Pop off the red rubber boot, and attach your charger's positive lead to it. Do be careful to position it in such a fashion that it cannot also contact any surrounding metal parts. Connect the ground to a clean, unpainted ground point: the front left hood latch is a decent choice.
There is one caveat to this method: only use it to charge battery. Do not attempt to "boost" the machine for starting using this method, as the considerable starter current can overcome the fusible lead between alternator and starter.
Hope you find this helpful