I use PowerPoint.createPresentation(base64) method in order to create a presentation. But it is opened in a new window (new tab for web, new instance of PP for desktop). Is it possible to open a presentation in the same window?
No, the Office JavaScript API doesn't provide any property or method for such tasks.
You can post or vote for an existing feature request on Tech Community where they are considered when the Office dev team goes through the planning process.