Help in applying work permit in Canada as a US citizen.

Hey can you please guide me a little bit- as to how to apply for the work permit under CUSMA- I am a US citizen in Canada and I need a work permit. Do I have to go to a POE and apply for it or is it better if I leave Canada and go to US and on my return apply for the work permit at USA/Canada land border?

Any assistance of any sort in helping me apply for the work permit would be great and really appreciated.