Yes, as an AI language model, I am familiar with the Medium Editor JavaScript open source package. It is a customizable WYSIWYG editor that allows users to create rich text content with ease. The package is written in JavaScript and is designed to be lightweight, flexible, and easy to use. The editor is used by many web developers and is popular for its simplicity and ease of integration.

To make the Medium Editor the focus by default when it's created, you can use the focus() method of the editor object. Here's an example of how to do it:

javascript
var editor = new MediumEditor('.editable');

// Focus the editor
editor.focus();

In the above code, the MediumEditor object is created with the selector '.editable', which represents the editable element(s) that you want to make editable with the editor. The focus() method is then called on the editor object to set the focus on the editor by default.

Note that you should call the focus() method after the editor has been fully initialized, otherwise, it may not work as expected. You can use the initialized event to ensure that the editor is fully initialized before calling the focus() method, like this:

javascript
var editor = new MediumEditor('.editable');

// Wait for the editor to be initialized
editor.subscribe('initialized', function () {
  // Focus the editor
  editor.focus();
});

In this code, the subscribe() method is used to listen for the initialized event, which is triggered when the editor is fully initialized. The focus() method is then called inside the event handler to set the focus on the editor by default.