Add meta box, print content and save_post. Read more: https://www.smashingmagazine.com/2011/10/create-custom-post-meta-boxes-wordpress/

Add Select Box To Wordpress

<?php
add_action('add_meta_boxes', 'add_country_name_meta_box');

function add_country_name_meta_box()
{
    add_meta_box(
        'country_name_meta_box',
        __('Widget Title', 'theme_name'),
        'add_country_name_meta_box_callback', // Prints the box content
        'post', // $page
        'normal', // $context
        'high' // $priority
    );
}

function add_country_name_meta_box_callback($post, $metabox)
{
    $value = esc_attr(get_post_meta($post->ID, 'country_name', true));

    wp_nonce_field(basename(__FILE__), 'country_name_nonce'); ?>

    <label for="country_name">
        <?php _e('Widget Label: ', 'theme_name'); ?>
    </label>

    <select name="country_name" id="country_name">
        <option value="" selected><?php _e('Please choose', 'theme_name'); ?></option>
        <option value="germany" <?php if($value == 'germany'): ?>selected<?php endif; ?>>Deutschland</option>
        <option value="usa" <?php if($value == 'usa'): ?>selected<?php endif; ?>>USA</option>
    </select> <?php
}

// Save post meta on the 'save_post' hook.
add_action('save_post', 'save_country_name', 10, 2);

function save_country_name($post_id, $post)
{
    /* Verify the nonce before proceeding. */
    if (!isset($_POST['country_name_nonce']) || !wp_verify_nonce($_POST['country_name_nonce'], basename(__FILE__)))
        return $post_id;

    /* Get the post type object. */
    $post_type = get_post_type_object($post->post_type);

    /* Check if the current user has permission to edit the post. */
    if (!current_user_can($post_type->cap->edit_post, $post_id))
        return $post_id;

    /* Get the posted data and sanitize it for use as an HTML class. */
    $new_meta_value = (isset($_POST['country_name']) ? sanitize_html_class($_POST['country_name']) : '');

    /* Get the meta key. */
    $meta_key = 'country_name';

    /* Get the meta value of the custom field key. */
    $meta_value = get_post_meta($post_id, $meta_key, true);

    /* If a new meta value was added and there was no previous value, add it. */
    if ($new_meta_value && '' == $meta_value)
        add_post_meta($post_id, $meta_key, $new_meta_value, true);

    /* If the new meta value does not match the old value, update it. */
    elseif ($new_meta_value && $new_meta_value != $meta_value)
        update_post_meta($post_id, $meta_key, $new_meta_value);

    /* If there is no new meta value but an old value exists, delete it. */
    elseif ('' == $new_meta_value && $meta_value)
        delete_post_meta($post_id, $meta_key, $meta_value);
}