thing.php

<?php

// Assuming the field name is 'url'.

// Don't forget to URL encode
$url = sprintf("http://www.facebook.com/share/dialog/app_id=xxxxxx%s&redirecht_uri=myurl.com", get_field('url'));

// Or use http_build_query
$url = sprintf('http://www.facebook.com/share/dialog/%s', http_build_query([
  'app_id' => 'xxxxxx',
  'value_name' => get_field('url'),
  'redirect_url' => 'myurl.com'
]);