I have just answered your support ticket on the subject, for convenience I will paste my answer here as well:
I am afraid that currently the desired functionality cannot be achieved out-of-the-box with he ImageEditor control. You can fire the DrawRectangle command on load of the control (imageEditor.executeCommand("DrawRectangle")), but the programmatic creation of a rectangle with specific coordinates will require a custom implementation.
If you decide to go through this road, you can extend the DrawRectangle.ascx dialog with the needed custom logic. More information on how to customize the built-in dialogs of the control is available in the following live demo: Customize Built-in Dialogs
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