Custom fields not visible in editor for custom post type

So I have successfully created a custom post type like this

add_action( 'init', 'create_post_type', 0 );

function create_post_type() {

    register_post_type( 'codingblog',
    // CPT Options
        array(
            'labels' => array(
                'name'          => __( 'CodingBlogs' ),
                'singular_name' => __( 'CodingBlog' ),
                'menu_name'     => __( 'My Coding Blog', 'blogus'),
                'all_items'     => __( 'All Coding Blogs', 'blogus'),
                'view_item'     => __( 'View Coding Blog', 'blogus'),
                'add_new_item'  => __( 'Add New Coding Blog', 'blogus'),
                'add_new'       => __( 'Add New', 'blogus'),
                'edit_item'     => __( 'Edit Coding Blog', 'blogus'),
                'update_item'   => __( 'Update Coding Blog', 'blogus'),
                'search_items'  => __( 'Search Coding Blog', 'blogus'),
                'not_found'     => __( 'Not Found', 'blogus'),
                'not_found_in_trash'    => __( 'Not found in Trash', 'blogus'),
            ),
            'label'             => __( 'codingblogs', 'blogus' ),
            'description'       => __( 'Coding Blog Posts', 'blogus'),
            'supports'          => array( 'title', 'editor', 'excerpt',
                                        'author', 'thumbnail', 'comments',
                                        'revisions', 'custom-fields', ),
            'taxonomies'        => array( 'post_tag', 'category', ),
            'hierarchical'      => false,
            'public'            => true,
            'show_ui'           => true,
            'show_in_menu'      => true,
            'show_in_nav_menus' => true,
            'show_in_admin_bar' => true,
            'menu_position'     => 5,
            'can_export'        => true,
            'has_archive'       => true,
            'exclude_from_search'       => false,
            'publicly_queryable'        => true,
            'capability_type'   => 'post',
            'rewrite'           => array('slug' => 'codingblogs'),
            'show_in_rest'      => true,
        )
    );
}

I was expecting (from what I read) that if I register a custom field with this custom type it will be visible in the editor when I edit an instance of this post type.

function create_post_type_fields() {
        register_post_meta( 'codingblog', 'themeslug_github_repo', [
                // Add to the REST response
                'show_in_rest'          => true,
                'single'                => true,
                'type'                  => 'string',
                'sanitize_callback'     => 'esc_url_raw',
        ] );
}

add_action( 'init', 'create_post_type_fields' );

But this has not worked. As I went to measure of including the ‘custom-fields’ in the supports list for the custom post type I would have expected this to work.

That said, when I have used a plugin for this purpose (CPTUI for example, with Advanced Custom Fields) I was forced to create a field group, and then add my custom field in the field group and then associate this with the custom post type. This is somehow divergent from the developer code examples I have found which don’t mention field groups at all.

So my questions are, why is the above not working? As I quite like field groups for the editor display, how can I create a custom post type, with one field group and one custom post type like the one above? I genuinely cannot find a code example for this use case after much searching.