//custom data tab
add_filter( 'woocommerce_product_data_tabs', 'add_my_custom_product_data_tab' , 99 , 1 );
add_action( 'woocommerce_product_data_panels', 'add_my_custom_product_data_fields',10,3 );
add_action( 'woocommerce_process_product_meta','woocommerce_process_product_meta_fields_save',10,2 );


function add_my_custom_product_data_tab( $product_data_tabs ) {
  $product_data_tabs['my-custom-tab'] = array(
    'label' => __( 'My Custom Tab', 'my_text_domain' ),
    'target' => 'my_custom_product_data',
  );
return $product_data_tabs;
}


function add_my_custom_product_data_fields() {
  global $woocommerce, $post;
     echo '<div id="my_custom_product_data" class="panel woocommerce_options_panel">';

    woocommerce_wp_textarea_input(
    array(
    'id'          => '_textarea[' . $post->ID . ']',
    'label'       => __( 'My Textarea', 'woocommerce' ),
    'placeholder' => '',
    'description' => __( 'Enter the custom value here.', 'woocommerce' ),
    'value'       => get_post_meta( $post->ID, '_textarea', true ),
  )
);

echo '</div>';
}


function woocommerce_process_product_meta_fields_save( $post_id ){

// Textarea
$textarea = $_POST['_textarea'][ $post_id ];
   if( ! empty( $textarea ) ) {
   update_post_meta( $post_id, '_textarea', esc_attr( $textarea ) );
 }

}

echo get_post_meta( $post->ID, 'my-field-slug', true );
 ex. echo get_post_meta( $post->ID, '_textarea', true );