Um so what is that plug called? I bought one without a power cable at all
I dont know the specs or part number for that connector; but reference online images for what the wire colors coming out are; and you know what pins you can solder to, or add pigtails with PCIE plugs on them.
If you doubt which half of the pins is + and which half is -; use an ohmmeter to check continuity to a board ground (use a capacitors polarity to find for instance).