Live example

by Сергій Куліков

HTML

<!-- Learn about this code on MDN: https://developer.mozilla.org/en-US/docs/Web/API/Element/copy_event -->

<div class="source" contenteditable="true">Try copying text from this box...</div>
<div class="target" contenteditable="true">...and pasting it into this one</div>

CSS

div.source, div.target {
    border: 1px solid gray;
    margin: .5rem;
    padding: .5rem;
    height: 1rem;
    background-color: #e9eef1;
}

JavaScript

const source = document.querySelector('div.source');

source.addEventListener('copy', (event) => {
    const selection = document.getSelection();
    event.clipboardData.setData('text/plain', selection.toString() + ' ВАСЯ');
    event.preventDefault();
});