“wpcargo_before_metabox_section” is an action hook that allows to add additional shipment form fields after the “Shipment Information” section in wp-admin

The following sample code will add custom “Shipment Notes” field, copy and paste to the theme functions.php file.

function my_metabox_custom_field(  ){
	global $post;
    // Get Save post metakey "_shipment_notes"
    $shipment_notes = get_post_meta( $post->ID, '_shipment_notes', true );
	?>
    <section class="col-md-6">
        <h3 class="section-title wpccf_section_header">Additional Information</h3>
        <div id="form-notes" class="form-group ">					
            <label for="_shipment_notes">Shipment Notes</label>
			<textarea id="_shipment_notes" class="md-textarea form-control _shipment_notes" name="_shipment_notes"><?php echo $shipment_notes; ?></textarea>
		</div>
	</section>
    <?php
}
add_action('wpcargo_before_metabox_section', 'my_metabox_custom_field');

Screenshot: