The ASRock B550 Steel Legend has two 12V Power Connectors for the CPU, on the right details about splitting the Corsair HX850i EPS12v CPU cable
The ASRock manual is not clear about these two 12V Power Connectors, labelled ATX12V1 (8-pins) and ATX12V2 (4-pins). For the ATX12V2 it only mentions; "Connecting an ATX 12V 4-pin cable here is optional". This means the 8-pins ATX12V1 is mandatory and is needed to power the CPU.
The 4-pins ATX12V2 provides additional power to the CPU, especially when overclocking, but is optional. Even when I do not plan to overclock I also connected ATX12V2 to guaranty a steady power supply to the CPU.
The Corsair HX850i High-Performance ATX Power Supply has an 8-pins EPS12V (also know as 8-pins ATX12V) CPU cable that can be split into two 4-pins connectors. If you want to connect the Power Supply into a 4-pins EPS motherboard port, simply pull the two halves of the connector apart
and connect one half into the motherboard port.