You can take a cheap $.99 extension cord and cut off the receptacle end and wire it directly to the pigtails on the fixture. The proper way to do it is to use
yellow wire nuts. If not, you can also just wrap them together tightly with electrical tape.
The polarized side of the plug (the wide one) is supposed to connect to the wire wire (neutral), and the narrow blade side connects to the black wire (hot). If you have the third safety ground plug on your extension cable, just strip it back and connect it to a screw on the frame of the light fixture if it doesn't have a bare copper or green wire available.