I just tried this with National today but they told me they could not add my Southwest number to the reservation because the system did not allow it. I think it is because I used autoslash/priceline to make the reservation.
Should I HUCA?
If not doable, then I need a Budget or National receipt prior to 2/23/19 to reinstate Southwest miles. Happy to pay you for it- let me know what you want.
Thanks!