Welcome to the world of Dell proprietary pinouts. The MH61R does not follow the Intel standard. At all.
You plugged in the standard Intel front panel header connector. You pressed the power button. Nothing happened. Dell Mih61r Mb Front Panel Pinout
You have a Dell OptiPlex 3010, 7010, or 390. The motherboard is the (often labeled CN-0Y8H8Y or similar). You decided to move this motherboard into a standard ATX case to save money, or perhaps you are trying to diagnose a power button failure. Welcome to the world of Dell proprietary pinouts
Do not trust the color of the wires on a Dell OEM case. Dell changes wire colors between revisions. You must go by pin position, not wire color. The Actual MH61R Pinout (Verified) Here is the pinout looking at the motherboard header with the PCIe slots facing down (bottom edge of the board towards you). You plugged in the standard Intel front panel
If you are reading this, you have likely just experienced a specific flavor of PC building frustration.