Adding a hidden checkout field in WooCommerce?

I want to include a link to a profile of the current user who submitted a checkout form through WooCommerce.

That is, to place automatically a current user’s author link like this in the hidden field: example.com/author/username

I want to achieve this by adding a hidden field in checkout form. So to get a link I would write something likes this:

<?php

$currentUser = get_current_user_id();

$user = get_user_by( 'id', $currentUser );

$userUrl = get_bloginfo( 'home' ) . '/author/' . $user->user_login;

echo $userUrl;
?>

My question is how can I create this type of hidden field in checkout form?

With a custom function hooked in woocommerce_after_order_notes action hook, you can also directly output a hidden field with this user "author link" as a hidden value, that will be submitted at the same time with all checkout fields when customer will place the order.

Here is that code:

add_action( 'woocommerce_after_order_notes', 'my_custom_checkout_hidden_field', 10, 1 );
function my_custom_checkout_hidden_field( $checkout ) {

    // Get an instance of the current user object
    $user = wp_get_current_user();

    // The user link
    $user_link = home_url( '/author/' . $user->user_login );

    // Output the hidden link
    echo '<div id="user_link_hidden_checkout_field">
            <input type="hidden" class="input-hidden" name="user_link" id="user_link" value="' . $user_link . '">
    </div>';
}

Then you will need to save this hidden field in the order, this way:

add_action( 'woocommerce_checkout_update_order_meta', 'save_custom_checkout_hidden_field', 10, 1 );
function save_custom_checkout_hidden_field( $order_id ) {

    if ( ! empty( $_POST['user_link'] ) )
        update_post_meta( $order_id, '_user_link', sanitize_text_field( $_POST['user_link'] ) );

}

Code goes in function.php file of your active child theme (or theme) or also in any plugin file.

The code is tested and working