ACF5 Pro licence activation. Could be adapted for any other plugin that requires a licence key, but doesn't yet support defining it in wp-config. Fires on theme activation.

licence activation.php

// Place this in wp-config
define( 'ACF_5_KEY', 'yourkeyhere' );

// Set ACF 5 license key on theme activation. Stick in your functions.php or equivalent.
function auto_set_license_keys() {
  
  if ( ! get_option( 'acf_pro_license' ) && defined( 'ACF_5_KEY' ) ) {
    
    $save = array(
		'key'	=> ACF_5_KEY,
		'url'	=> home_url()
	);
	
	$save = maybe_serialize($save);
	$save = base64_encode($save);
      
    update_option( 'acf_pro_license', $save );
  }
}
add_action( 'after_switch_theme', 'auto_set_license_keys' );

Have you ran into any issues with this when switching domains (e.g. local > staging > prod)?

It seems ACF checks to make sure the home_url that you saved is still the same as the current home_url, and so functionality to check for that and fix it if it's different is required.

Tried it but does not seems to work?

This is my version using @elliotcondon original code. It fires only on backend.
Please add it to your functions.php on your needs.

<?php
class AutoActivator {
		
		const ACTIVATION_KEY = 'youractivationkeyhere';
		
		/**
		 * AutoActivator constructor.
		 * This will update the license field option on acf
		 * Works only on backend to not attack performance on frontend
		 */
		public function __construct() {
			if (
				function_exists( 'acf' ) &&
			    is_admin() &&
				!acf_pro_get_license_key()
			) {
				acf_pro_update_license(self::ACTIVATION_KEY);
			}
		}
		
	}
       
       new AutoActivator();
?>

My version, filtering on the get_option function for that specific option_name https://gist.github.com/polevaultweb/9cdf1b3bfeb054f13f221d4612bd8901

My version, filtering on the get_option function for that specific option_name https://gist.github.com/polevaultweb/9cdf1b3bfeb054f13f221d4612bd8901

You should remember to give everybody know to add this in wp-config.php. Then it will work correctly.

// Place this in wp-config
define('ACF_PRO_LICENSE', 'yourkeyhere' );