This is the easiest specialist mission and nearest one from your starting position so completing this as early as you can will help you a lot in the long run. Besides, Boomer’s unique skills are quite essential as well for Stealth liberations or assassinations.
Head to Rae-rae’s Pumpkin Farm which is impossible to miss since it’s already marked in the map. You can approach it from the hill on the farm’s eastern side to give you a vantage point to scout the farm below. You should see three cultists harassing a dog inside a cage. You don’t need to be stealthy and these are basic, easy-to-kill cultists so there’s no problem in going wild against them. After getting rid of them, approach the cage and hit the gate to free Boomer. Pet him to get the good boy’s trust and become a permanent part of your roster.