As soon as the Lustrous Odyssey event goes live, you can hatch or encounter Dusk Rockruff in Pokemon GO. The important thing to look forward to is whether the caught Rockruff can evolve into Dusk Lycanroc.
You can hatch 2 KM or 7 KM eggs in Pokemon GO to have a chance to get Dusk Rockruff. To clarify, not all Rockruff you will hatch can evolve into Dusk Form Lycanroc. Therefore, the strategy you’ll need to apply would be to hatch as many eggs as possible.
Unfortunately, you cannot find Rockruff in Pokemon GO wild encounters, but you can acquire a few PvP catches for your battle team.
Completing the Field Research is another way to get a Dusk Rockruff encounter. It’s worth noting only some tasks reward you with the encounter. Most of the caught ones will either evolve into Midnight or Midday Form.
Developer Niantic's official announcement did not clarify that Dusk Rockruff can be caught from a raid. Surprisingly, many Pokemon GO players from different parts of the world have seen it appearing in one-star raids.