The pcie slot only has 12v and 3.3v and a few detection rails on the power side, so i suspect that there is a 12v to 5v buck converter on the card to provide the power, but it won’t hurt to try the 5v first. Worst that can happen is that the card doesn’t power on due to too low power, after which you can try 12v if necessary so as not to blow up the card. Also depending on how much power your pcie device needs, make sure to go with an m.2 to pcie that has the proper cabling and connector as anything else can be a fire risk.