successful_befriend_potential_friend_effect

successful_befriend_potential_friend_effect = {
	$CHARACTER$ = {
		if = { #Always leads to a potential friendship
			limit = {
				NOR = {
					has_relation_potential_friend = root
					has_relation_friend = root
				}
			}
			if = { #Remove potential rival relation if you have it
				limit = {
					has_relation_potential_rival = root
				}
				remove_relation_potential_rival = root
			}
			set_relation_potential_friend = root
		}
		if = {
			limit = {
				number_of_traits_in_common = {
					target = root
					value >= 3
				}
			}
			$CHARACTER$ = {
				add_opinion = {
					modifier = friendliness_opinion
					target = root
					opinion = 30
				}
			}
		}
		else = {
			$CHARACTER$ = {
				add_opinion = {
					modifier = friendliness_opinion
					target = root
					opinion = 20
				}
			}
		}
	}
}

As written in game/common/scripted_effects/00_scheme_scripted_effects.txt.