supportsExternalLinks example

      $element['uri']['#target_type'] = 'node';
      // Disable autocompletion when the first character is '/', '#' or '?'.       $element['uri']['#attributes']['data-autocomplete-first-character-blacklist'] = '/#?';

      // The link widget is doing its own processing in       // static::getUriAsDisplayableString().       $element['uri']['#process_default_value'] = FALSE;
    }

    // If the field is configured to allow only internal links, add a useful     // element prefix and description.     if (!$this->supportsExternalLinks()) {
      $element['uri']['#field_prefix'] = rtrim(Url::fromRoute('<front>', [], ['absolute' => TRUE])->toString(), '/');
      $element['uri']['#description'] = $this->t('This must be an internal path such as %add-node. You can also start typing the title of a piece of content to select it. Enter %front to link to the front page. Enter %nolink to display link text only. Enter %button to display keyboard-accessible link text only.', ['%add-node' => '/node/add', '%front' => '<front>', '%nolink' => '<nolink>', '%button' => '<button>']);
    }
    // If the field is configured to allow both internal and external links,     // show a useful description.     elseif ($this->supportsExternalLinks() && $this->supportsInternalLinks()) {
      $element['uri']['#description'] = $this->t('Start typing the title of a piece of content to select it. You can also enter an internal path such as %add-node or an external URL such as %url. Enter %front to link to the front page. Enter %nolink to display link text only. Enter %button to display keyboard-accessible link text only.', ['%front' => '<front>', '%add-node' => '/node/add', '%url' => 'http://example.com', '%nolink' => '<nolink>', '%button' => '<button>']);
    }
    // If the field is configured to allow only external links, show a useful     // description.     elseif ($this->supportsExternalLinks() && !$this->supportsInternalLinks()) {