The two pins that I can connect a power button. The only button I can see the BIOS reset button which restarts the CPU but also resets the BIOS to default.
The Onda D1800 is configured from the factory to boot automatically when power is applied; perhaps the B250 is configured similarly. Did you try doing that?
Otherwise, the pin header for the usual power switch, power LED, reset switch, HDD LED should be located in the corner near the ATX 24 pin connector.
my onda b250 d8p v1 has this pinout for the front panel header. i assume the other onda mobos have the same pinout and colors.