Programmatically add coupon code to WooCommerce checkout

auto-add-woocommerce-coupon.php

<?php

// Automatically add a WooCommerce coupon code 
// Currently added when headed to checkout, but can be connected to different hook.

add_action('woocommerce_before_checkout_form', function() {
    global $woocommerce;
    $coupon_code = 'COUPON_CODE'; // Replace with your code
    
    // Add additional logic if necessary
    
    if ( ! $woocommerce->cart->add_discount( sanitize_text_field( $coupon_code )) ) :
        $woocommerce->show_messages();
    endif;

});