While both Rockruff and Lycanroc are available in Pokémon Sword and Shield’s Isle of Armor expansion, Dusk Form Lycanroc is a little more elusive.
As of now, the only confirmed way to obtain Dusk Form Lycanroc in Sword and Shield is by transferring it over through Pokémon Home from Ultra Sun and Ultra Moon.
This might not be the only way to get the rarest form of Lycanroc, but actually obtaining it while traveling the Isle of Armor will take a lot of work. Outside of hunting down a Rockruff, you’ll also need to make sure you catch one with the ability Own Tempo, which is considered a Special Ability, not a Hidden one.
If you manage to find one with that ability, which might not be possible in Sword and Shield, you’ll need to either wait or set your system time to one of two very specific times of day.
Rockruff only evolves into Dusk Form Lycanroc between 5pm and 6pm local time (Ultra Sun) or 5am and 6am local (Ultra Moon). No one has had a chance to test those windows with the right Rockruff yet, but if neither of those factors work, then the only way to get the third Lycanroc form will be through Home.
Game Freak might release a special Rockruff capable of evolving into Dusk Form Lycanroc through Mystery Gift or Max Raid Battles in the future, but that won’t help the players who want to add it to their teams right now.