I am not sure what 240 plug/outlet you have.
NEMA 6-50 uses three wires. Hot on the two black screws left and right. The Green wire goes on the green screw which is on the top. In this configuration you do not use the white. It's really a neutral wire. You can tell you hot wires by measuring with a volt meter black to white = 120 volt red to white = 120 volt or substitute the green to white, you get the same voltage. So just wire nut the white wire off. It's not used in this configuration.
NEMA 14-50R you will need to wire the two hot wires (120 Volt) on the dark colored screws. Usually Black and another color like red. The green is the ground and will go on the green lug. The white is on the bottom.
check this out:
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