Straight from the EW article posted here.
“The crossover — which will air on Thursday, March 29 on The CW for the rest of the world — was first announced at the CW Upfronts in May 2017. And for the most part, details of the episode have been kept under wraps. At this point, all we really know is what co-showrunner Andrew Dabb previously told EW: “It’s more adult than your average episode of Scooby-Doo.”
As for its PaleyFest premiere, the episode is set to air before members of Supernatural‘s cast and creative team take the stage for a panel. Tickets to the event are available on the official PaleyFest website.”