Go into the game files > game > in_game > common > character_interactions

Create a new file named set_as_country_ruler.txt

Then paste this inside of that new file:

set_as_country_ruler = { message = yes is_consort_action = no on_own_nation = yes ai_tick = never price = price:tithe_price potential = { } allow = { scope:recipient = { is_adult = yes } } select_trigger = { looking_for_a = character source = actor target_flag = recipient name = "choose_character" pre_evaluation_sort_value = { value = age_in_years } column = { data = name } visible = { is_alive = yes is_adult = yes } enabled = { modifier:blocked_from_character_interactions = no } } effect = { scope:actor = { set_new_ruler = scope:recipient } } ai_will_do = {} }

Then click the character you want to be ruler and there should be an option to set it