I'm not aware of a way to sign up for a route notification from Southwest. The March 4th press release simply stated "Additional service details, including plans for previously announced gateways of San Diego and Sacramento, and for Lihue, on Kauai, will be announced in the coming weeks." I'm guessing that the unannounced internal timing be be delayed given the grounding of the MAX 8. Southwest was only using the -800 on the Hawaii routes but they've had to reshuffle their entire flight schedule and right now they're having a hard enough time taking care of all booked passengers.
The best way to get notified as soon as possible may be to set up a news alert on a third party site or news site. For example Google has instructions on how to "Create an alert".
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