By "bookmark," do you mean a Link to Another Site?
If that is the case, edit one of the text capsules on your hub. Select a word or group of words that you want to be the link and click on the icon that looks like a "paper clip." Then, you can simply add the webpage address(URL) to create a link.
You can also add a Link Capsule, which is the capsule that is shaped like a "paper clip" located under the "Add More Capsules" menu in the upper right hand corner of the hub.