Converting Hard Wire Fixture to Plug In
Some very important information is missing here. Where are you intending to place that lighting fixture?

Normally a light like that is mounted in a permanent manner to a wall. There should be an electrical box in the wall that is positioned directly behind the hole shown here:



Three wires from the fixture, white, black and a safety ground pass through that hole and connect to the permanent wiring in the electrical box. You may want to consider adding an additional safety ground wire from that green screw showing in the above picture. 

You will not use that two prong cord suggested by the person at Home Depot. They do not know what they are talking about. You do need to have the safety ground wire of the fixture connected into the safety ground of the building wiring.