First, google how to spatchcock a chicken. Follow instructions. Place the spatchcocked chicken in a pan enough to cover water and brine. In a bowl, whisk together salt, sugar, and beer.
Pour the brine over the chicken and then pour water to cover the whole thing. Wrap the pan with saran wrap tightly. Carefully transfer into the fridge to marinate for 3-4 hours or overnight.
Prepare your pellet grill accordingly.
Take the chicken out from the brine solution and pat dry with paper towels. Rub both sides of chicken with the dry-rub seasoning of your choice.
Once the pellet grill is ready, place the chicken in the grill to cook. Follow heating and cooking instructions from your grill's pamphlet.