create method
Creates a reaction and adds it to a message.
Only unicode emojis are supported. For an example, see Add a reaction to a message. Requires user authentication.
request - The metadata request object.
Request parameters:
parent - Required. The message where the reaction is created. Format:
spaces/{space}/messages/{message}
Value must have pattern ^spaces/\[^/\]+/messages/\[^/\]+$.
$fields - Selector specifying which fields to include in a partial
response.
Completes with a Reaction.
Completes with a commons.ApiRequestError if the API endpoint returned an error.
If the used http.Client completes with an error when making a REST call,
this method will complete with the same error.
Implementation
async.Future<Reaction> create(
  Reaction request,
  core.String parent, {
  core.String? $fields,
}) async {
  final body_ = convert.json.encode(request);
  final queryParams_ = <core.String, core.List<core.String>>{
    if ($fields != null) 'fields': [$fields],
  };
  final url_ = 'v1/' + core.Uri.encodeFull('$parent') + '/reactions';
  final response_ = await _requester.request(
    url_,
    'POST',
    body: body_,
    queryParams: queryParams_,
  );
  return Reaction.fromJson(response_ as core.Map<core.String, core.dynamic>);
}