This is how I add a cron action in a WordPress Plugin #wp

cron-example.php

<?php
/*
Plugin Name: My Cronjobs
*/

defined('ABSPATH') or die;

/**
*   Add Cron when the theme is activated
*/
register_activation_hook(__FILE__, function () {
	wp_schedule_event(time(), 'hourly', 'my_hourly_event');
});

/**
*   Cron action
*/
add_action('my_hourly_event', function () {
	
});

/**
*   Remove Cron when the theme is deactivated
*/
register_deactivation_hook(__FILE__, function () {
	wp_clear_scheduled_hook('my_hourly_event');
});