How do I display a custom field in an existing form on the front end?

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This worked for me:

In the template file (used with that existing form), before the get_header(); call/line, add this code:

global $my_dates_MB;

// If you changed the meta box ID (i.e. mydates), change it also below.
add_filter( 'rwmb_show_mydates', '__return_false' );

$meta_boxes = get_dates( [] );
$my_dates_MB = rwmb_get_meta_box( $meta_boxes[0] );

if ( isset( $_POST['my_dates_post_ID'] ) ) {
    // Save the meta data.
    $my_dates_MB->save_post( $_POST['my_dates_post_ID'] );
}

// Enqueue styles and scripts.
add_action( 'wp_enqueue_scripts', [ $my_dates_MB, 'enqueue' ] );
//$my_dates_MB->enqueue();

Then replace this:

$mydates = if(isset($_POST['prefix-mydates'])) echo $_POST['prefix-mydates'];


<fieldset>
<input type="text" value=""  id="prefix-mydates" class="rwmb-date hasDatepicker" size="60" tabindex="20" name="prefix-mydates">
</fieldset>

..with this:
(UPDATED; May 02 2018)

<?php
global $my_dates_MB;

// Get the current post ID.
$post_id = get_the_ID();

// Set the post ID for the "meta box".
$my_dates_MB->set_object_id( $post_id );

// Show the meta box.
$my_dates_MB->show();
?>

<input type="hidden" name="my_dates_post_ID" value="<?php echo $post_id; ?>" />

That will print the whole “meta box” as defined in your custom get_dates() function (btw, change its name to something unique), but that should not be an issue since the meta box has just one field.

You’d get something like this: (not including the “Submit” button)

enter image description here