How to prevent employee from pasting image in rich text editor in Success Criteria of Goal

Summary:

Customer wants to disable image pasting in Success Criteria of goal in Me -> Career & Performance -> Goals

Content (please ensure you mask any confidential information):

Currently users are pasting images i.e. screenshots in Goals as rich text editor 'CKEditor' does not prevent pasting images, I've tried to use javascript which prevents pasting of images in textbox area.

document.getElementById('call-text-form-textarea').addEventListener('paste',function(e) {
    e.preventDefault();
});

However question is how to add custom javascript.

Version (include the version you are using, if applicable):

23D

Code Snippet (add any code snippets that support your topic, if applicable):