RTX 5090 in Dell Pro Max Tower T2 (FCT2250)

Hi all,

I got a FCT2250 with a RTX 5070 installed, and trying to upgrade to an RTX 5090.

The provided cable however, is not enough to power the latter (the cable plugs into 1x6-pin and 1x8-pin headers of the PSU; out of 2x6-pin +2x8-pin available). 

Anybody has an advice with running a 600W card on this system? Since I don't know the pinout config out of the PSU, I don't know if any of the configs for other PSUs (see e.g. https://store.cablemod.com/product-category/cablemod-basics/cb-12vhpwr/) might work ...