If the PSU lacks the proper connections they is likely not going to power the card reliably with an adapter. System specs including the make and model of the PSU would be helpful here.
Power Supply:XFX TS 650W 80+ Gold Certified ATX Power Supply (£71.28 @ Amazon UK) Total: £71.28 Prices include shipping, taxes, and discounts when available Generated by PCPartPicker 2016-08-08 23:52 BST+0100
Power Supply:XFX 650W 80+ Bronze Certified ATX Power Supply (£50.51 @ CCL Computers) Total: £50.51 Prices include shipping, taxes, and discounts when available Generated by PCPartPicker 2016-08-09 00:11 BST+0100