gistfile1.js

var moment = {
  'type' : 'http://schemas.google.com/AddActivity',
  'target' : {
    'url' : 'https://developers.google.com/+/web/snippet/examples/widget'
  }
};

var request = gapi.client.request({
  'path' : 'plus/v1/people/me/moments/vault',
  'method' : 'POST',
  'body' : JSON.stringify(moment)
});

request.execute(function(result) {
  console.log(result);
});