Easily and quickly add a new custom field to posts and pages by using the Add custom field option.

You can have a pre-set in there too by adding this code to functions.php

Source: https://www.thewebtaylor.com/articles/wordpress-adding-default-custom-fields-on-new-posts

add_action('wp_insert_post', 'set_default_custom_fields');
function set_default_custom_fields($post_id){
if ( $_GET['post_type'] == 'post' ) {

add_post_meta($post_id, 'Field Name', '', true);
add_post_meta($post_id, 'Another Field Name', '', true);
}
return true;
}

Add the custom field to the template file

Source: http://www.wpbeginner.com/wp-tutorials/wordpress-custom-fields-101-tips-tricks-and-hacks/

<?php echo get_post_meta($post->ID, 'key', true); ?>

custom-field-name is the name of your new function (case sensitive)