it's in module_dialogs, more specifically:
[anyone,"lord_ask_enter_service", [(troop_get_slot, ":renown", "trp_player", slot_troop_renown),
(store_mul, ":vassal_potential", "$g_talk_troop_relation", 5),
(val_add, ":vassal_potential", ":renown"),
(ge, ":vassal_potential", 160),
(call_script, "script_get_poorest_village_of_faction", "$g_talk_troop_faction"),
(gt, reg0, 0),
(assign, "$g_invite_offered_center", reg0),