I have a Battery Tender, something similar to the one you mention. It too has the pigtail which gets attached to the battery. When using the pigtail, it's just like attaching the clamps from the charger directly to the battery itself. Did the one you have come with the pigtail cord?
If so, just hook the pigtail up to the battery (red to positive, black to negative) and plug the charger into that.
AKA: Tim & 'The Adventure Cycle' VROC #24567, NEVROC, SteelCity VROC
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