How to add a custom attribute?

Question

How to add a custom attribute in the field Contact Form 7 without javascript ?

For example, there is such a field on the page:

Answer 1

Extending on from Tofandel's solution, for the benefit of those who got 99% of the way there, but suffered validation issues - I've resolved that in my case and would like to offer an extended solution that gets as far as Tofandel's (putting the attribute into the form proper) but also successfully validates on submission.

    add_filter('wpcf7_form_tag', function($tag) {
      $data = [];
      foreach ((array)$tag['options'] as $option) {
        if (strpos( $option, 'autocomplete') === 0) {
          $option = explode(':', $option, 2);
          $data[$option[0]] = apply_filters('wpcf7_option_value', $option[1], $option[0]);
        }
      }
      if(!empty($data)) {
        add_filter('wpcf7_form_elements', function ($content) use ($tag, $data) {
          $data_attrs = wpcf7_format_atts($data);
          $name = $tag['name'];
          $content_plus_data_attrs = str_replace("name=\"$name\"", "name=\"$name\" " . $data_attrs, $content);

          return $content_plus_data_attrs;
        });
      }
      return $tag;
    } );

Rather than changing the tag ID to a random value only to replace it with the "real" value later, we just reference the real value in the first place, replacing the relevant part of the content in the wpcf7_form_elements filter (in my case, autocomplete, but as Tofandel's example shows, this can be extended to any data attribute you'd like).