You can achieve this by writing some custom jQuery / PHP. If you are not familiar with writing custom jQuery / AJAX / PHP scripts, you may need to contract a developer to do this for you.
Upon ‘change’ of the post object field, you will need to perform a custom AJAX query to get the selected post’s content.
With the post’s content returned, you will then need to use JS to update the WYSIWYG field. This is not the easiest task, but is possible by reading over the
js/input.js file and scrolling down to the WYSIWYG section. You will see many tinymce functions which you can use to update the value.
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