In small bowl, combine agave, ketchup, vinegar, horseradish, garlic and red pepper flakes until well blended. Reserve half of sauce. In separate bowl, mix together remaining ingredients, except buns. Blend in 2 tablespoons of reserved sauce. Divide salmon mixture into 4 patties, 1/2- to 3/4-inch thick. Place patties on well-oiled grill 4 to 6 inches from hot coals. Grill, turning 2 to 3 times and basting with sauce, until burgers are browned and heated through. Place on hamburger buns and serve with reserved sauce.