Add Additional Fields to WP Post Type Meta

gistfile1.php

<?php
function wpptm_add_select_input( $settings ) {
    
    /* add our setting to the settings array */
    $settings[] = array(
        'type'=> 'select',
        'label'=> 'A Select Box',
        'id'=> ‘selectbox’,
        'options' = array(
            array(
                'name' => 'Display Name One',
                'value' => '1'
            ),
            array(
                'name' => 'Display Name Two',
                'value' => '2'
            )
        )
        'class'=> 'select_box_css_class'
    );
    
    /* return the modified settings array */
    return $settings;
    
}

add_filter( 'wpptm_settings', 'wpptm_add_select_input', 20 );