Not all people can apply SOWP at the border. It only for people who doesn't need a visa, or with eTA. Also depends on the border too, Idk which part you're living now.
For us, we did at Kingston border. Entered the US, going around, having lunch. When stop at Canadian side, tell CBSA officer that you want to apply for a wp. Then they will send you to the office. Remember to bring your paperwork (I brought the whole PR package and printed AOR email).
Another officer will ask a few questions (Which I don't really know which they will ask), then you sit and wait. Then they call you to pay the fee ($255 + 85 for fingerprint). Then they give you the OWP.
After being back, go to the webform to let them now that you already have a work permit, then they will withdraw the online submission and refund (they told me they would refund within 16 weeks)
Good luck! Hope it helps