php - How to fill custom checkout field with previously entered value, like default WooCommerce checkout fields?

Solution:

Updated (replaced wrongWC_Session methodset() withget() on the first function)

This will work for guest users too. Replace your code with:

// Display checkout custom field
add_action( 'woocommerce_before_order_notes', 'add_custom_checkout_field' );
function add_custom_checkout_field( $checkout ) { 
    $key_field = 'gst_no';
       
    woocommerce_form_field( $key_field, array(        
        'type'        => 'text',        
        'class'       => array( 'form-row-wide' ),        
        'label'       => __('GST Number'),        
        'placeholder' => __('GST Number'),        
        'required'    => true
        //'default'   => $saved_gst_no,        
    ), $checkout->get_value($key_field) ? $checkout->get_value($key_field) : WC()->session->get($key_field) ); 
}

// Save checkout custom field value in a WC_Session variable 
add_action( 'woocommerce_checkout_create_order', 'action_checkout_create_order', 10, 2 );
function action_checkout_create_order( $order, $data  ) {
    $key_field = 'gst_no';
    
    if( isset($_POST[$key_field]) ) {
        WC()->session->set($key_field, sanitize_text_field($_POST[$key_field]));
    }
}

// Save checkout custom field value as user meta data
add_action( 'woocommerce_checkout_update_customer', 'action_checkout_update_customer', 10, 2 );
function action_checkout_update_customer( $customer, $data  ) {
    $key_field = $key_field;
    
    if( isset($_POST['gst_no']) ) {
        $customer->update_meta_data($key_field, sanitize_text_field($_POST[$key_field]));
    }
}
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Note: Uses a WC_Session variable to store the submitted value for guests, allowing to display it on checkout when returning without completing the transaction.

Solution:

In my experience, WooCommerce was saving the form data in session variables via AJAX when fields were updated via theupdate_order_review path. In order to hook into this and stored my custom vars, I used the following:

add_action('woocommerce_checkout_update_order_review', 'custom_woocommerce_checkout_update_order_review');
function custom_woocommerce_checkout_update_order_review($post_data){

    // Convert $post_data string to array and clean it
    $post_arr = array();
    parse_str($post_data, $post_arr);
    wc_clean($post_arr);

    if(isset($post_arr['gst_no'])) {
        WC()->session->set('gst_no', $post_arr['gst_no']);
    }
}
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Also a deeper way to add the custom checkout fields so that it's available in the $data passed towoocommerce_checkout_update_order_meta:

add_filter('woocommerce_checkout_fields', 'custom_checkout_fields');
function custom_checkout_fields($fields){
    $fields['gst_no'] = array(
        'type'        => 'text',
        'default'     => WC()->session->get('gst_no')
    );
    return $fields;
}
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