If your replacement power cord doesn't come with an anchor for the electrical box:

Remove the anchor from the original cord by pulling on it with a strong, steady force. You may need to use pliers or other gripping tools.

Once the anchor is free, remove the rubber stopper from inside it with a pair of needle nose pliers.

Finally, slide the empty anchor onto the new power cord, followed by the stopper. Then slide the anchor up onto the stopper until it can no longer be easily moved.