How/where do I connect PCI-E or Molex power in a A1188 2008 Mac Pro tower?

On my workbench I have a 2008 Mac Pro tower A1188. According to this document I ordered a AMD Radeon RX 560 (this one) to replace a faulty graphics card.

The RX 560 fits on the board, but it has a connector to get more power from the power supply. It appears to be a 6 pin PCI-E female connector on the GPU, and in the box was a short adapter cable that plugs into that PCI-E slot on the GPU, with two Molex connectors on the other end.

The problem is, I do not see any connections on the Mac Pro's motherboard or power supply for Molex or PCI-E. I am not really a Mac expert, but this seems crazy because every PC I've ever worked on has Molex cables hanging around.

How do I get this GPU connected?