Typically a selection (2 or 3) of power cables ship with the HDX card. Sonnet i think has a black 6-pin female cluster in a 2x3 config with a latch, the card has that black female 4x1 config (also as a triple daisy chain to connect up to HDX3). There's some alternatives to connect to PC 4-pin disk drive power etc...
The pix below show the included with HDX card standard power cable and an Avid TB3 rack mount chassis (which is Sonnet). And that works fine. Not sure, where to source them best, saw some overpriced on eBay upon googling "HDX card power cable". Attached the part number sticker from their posting if you want to try to get it from Avid as a spare. Or if anybody on here bought 2 HDX cards, they get 2 cables but in an HDX2 system of course only need one...
Their recommended cable appears to be the result of a change with a particular Sonnet eGFX chassis, so they thought you might need that cable (assuming you had the one that came with the card and not being able to use it).
https://www.pro-tools-expert.com/hom...erbolt-chassis