Custom Field 101

I would not use custom fields or shortcodes for this functionality, I would look at filters. the_content filter and the loop_end action (just make sure that your theme uses while ( have_posts() ) in single post pages) comes to immediate mind here. These are two options which requires no modification of any of your files in your theme. This way you can add extra content to your single post pages (which I assume is what you need) without having to create a child theme.

Approaching your problem this way, you also do not need to remember to use the shortcode or set the custoim field for a particular post, so you have a more dynamic way which you set once and forget about it

For conveniency, and also the recommended way, is to create your own custom plugin in which you can dump the code. This way, even if you switch themes, your custom text will still show without having to modify the new theme or create yet another child theme.

Lets look at the two methods to add your custom text line: (NOTE: All code is untested and should go into a custom plugin)

the_content FILTER

This will add your custom text after the content on your single post page

add_filter( 'the_content', function ( $content )
{
    /** 
     * Only filter the content on the main query, and when the post belongs
     * to the washington category and are tagged county. Adjust as needed
     */
    if ( !is_single() )
        return $content;

    if ( !in_the_loop() )
        return $content;

    $post = get_queried_object();
    if (    !in_category( 'washington', $post ) 
         && !has_tag( 'county', $post )
    )
        return $content;

    // We are inside the main loop, and the post have our category and tag
    // Get the post title
    $MyCounty = apply_filters( 'the_title', $post->post_title );
    // Set our custom text
    $text = "If you live in $MyCounty, tell us what you think.";

    return $content . $text;
}):

loop_end ACTION

This will add your text after the post, you can also use loop_start to add the text before the loop, or even the_post which will also add the text before the post. Just note, all actions needs to echo their output

add_action( 'loop_end', function ( \WP_Query $q )
{
    // Only target the main query on single pages
    if (    $q->is_main_query()
         && $q->is_single()
    ) {
        $post = get_queried_object();
        if (    in_category( 'washington', $post )
             && has_tag( 'county', $post )
        ) {
            $MyCounty = apply_filters( 'the_title', $post->post_title );
            $text     = "If you live in $MyCounty, tell us what you think.";

            // Echo our custom text
            echo $text;
        }
    }
});