Have Military Governor Join War

is_available = OR = {
	AND = {
		government_allows = merit
		top_liege ?= this
		government_has_mechanic = administrative
	}
	has_title = title:e_minister_of_war
}
trigger_if = {
	limit = {
		is_ai = yes
	}
	OR = {
		AND = {
			government_allows = merit
			top_liege ?= this
			tgp_has_access_to_ministry_trigger = no
			government_has_mechanic = administrative
		}
		has_title = title:e_minister_of_war
	}
}
is_shown = scope:recipient = {
	top_liege ?= scope:actor.top_liege
	OR = {
		is_landed = yes
		has_title = title:e_minister_grand_marshal
	}
}

As written in game/common/character_interactions/10_tgp_interactions.txt.

custom_tooltip = {
	text = celestial_request_military_aid_no_military_governors_desc
	scope:recipient.top_liege = {
		any_vassal_or_below = {
			OR = {
				vassal_contract_has_flag = celestial_military_appointment
				vassal_contract_has_flag = meritocratic_military_appointment
			}
		}
	}
}
custom_tooltip = {
	text = celestial_request_military_aid_no_available_military_governors_desc
	scope:recipient.top_liege = {
		any_vassal_or_below = {
			tgp_celestial_military_governor_can_join_war_trigger = {
				TARGET_GOVERNOR = scope:recipient
				MINISTER_OF_WAR = scope:actor
			}
		}
	}
}
scope:recipient = {
	is_at_war = yes
	is_at_war_with_liege = no
	NOT = {
		is_at_war_with = scope:actor
	}
	trigger_if = {
		limit = {
			is_at_war = yes
		}
		custom_description = {
			text = target_is_not_war_leader
			object = scope:recipient
			any_character_war = {
				scope:recipient = {
					is_leader_in_war = prev
				}
			}
		}
	}
}

As written in game/common/character_interactions/10_tgp_interactions.txt.

on_send = 
on_accept = scope:recipient = {
	save_scope_as = governor_at_war
	scope:secondary_recipient = {
		save_scope_as = governor_joining
	}
	every_character_war = {
		limit = {
			scope:recipient = {
				is_leader_in_war = prev
			}
			is_attacker = scope:recipient
		}
		add_attacker = scope:secondary_recipient
	}
	every_character_war = {
		limit = {
			scope:recipient = {
				is_leader_in_war = prev
			}
			is_defender = scope:recipient
		}
		add_defender = scope:secondary_recipient
	}
	if = {
		limit = {
			is_ai = yes
		}
		add_opinion = {
			target = scope:actor
			modifier = grateful_opinion
			opinion = 10
		}
	}
	if = {
		limit = {
			scope:actor != scope:recipient
		}
		trigger_event = {
			id = tgp_interaction_event.0015
		}
	}
}
scope:secondary_recipient = {
	trigger_event = {
		id = tgp_interaction_event.0016
	}
}
scope:actor = {
	hidden_effect = {
		send_interface_toast = {
			type = event_toast_effect_neutral
			title = celestial_send_military_aid_confirmation
			left_icon = scope:recipient
			right_icon = scope:secondary_recipient
			custom_tooltip = celestial_send_military_aid_effect_desc
		}
	}
}
on_decline = 

As written in game/common/character_interactions/10_tgp_interactions.txt.