Copy and paste the following code to your theme functions.php file

Available filters in the registration form templates

• wpcfe_personal_info_fields
• wpcfe_billing_address_fields

// Custom Code
function my_custom_registration_field( $fields ){
	// Add custom field Unique ID
	$custom_field = array(
		'_unique_id'     => array(
			'id'			=> '_unique_id',
            'label'			=> __('Unique ID', 'wpcargo-frontend-manager'),
            'field'			=> 'text',
            'field_type'	=> 'text',
            'required'		=> false,
            'options'		=> array(),
			'field_data'	=> array(),
			'field_key'		=> '_unique_id'
        )
	);
	return array_merge($custom_field, $fields );
}
add_filter( 'wpcfe_personal_info_fields', 'my_custom_registration_field');