Code for post https://barrysimpson.net/posts/copy-paste-chrome-ext

## background.html
<!DOCTYPE html>
<html>
<head>
	<meta charset="utf-8">
	<script src="background.js"></script>
</head>
<body>
	<textarea id="ta"></textarea>
</body>
</html>

## background.js
'use strict';

function getClipboard() {
	var result = null;
	var textarea = document.getElementById('ta');
	textarea.value = '';
	textarea.select();

	if (document.execCommand('paste')) {
		result = textarea.value;
	} else {
		console.error('failed to get clipboard content');
	}

	textarea.value = '';
	return result;
}

function setClipboard(value) {
	var result = false;
	var textarea = document.getElementById('ta');
	textarea.value = value;
	textarea.select();

	if (document.execCommand('copy')) {
		result = true;
	} else {
		console.error('failed to get clipboard content');
	}

	textarea.value = '';
	return result;
}

chrome.runtime.onMessageExternal.addListener(function(request, sender, sendResponse) {
	switch (request.method) {
		case 'getClipboard':
			sendResponse(getClipboard());
			break;
		case 'setClipboard':
			sendResponse(setClipboard(request.value));
			break;
		default:
			console.error('Unknown method "%s"', request.method);
			break;
	}
});

## index.html
<!DOCTYPE html>
<html>
<head>
	<meta charset="utf-8">
	<meta http-equiv="X-UA-Compatible" content="IE=edge">
	<title></title>
	<meta name="description" content="">
	<meta name="viewport" content="width=device-width">
</head>
<body>
	<h4>Current Clipboard Text:</h4>
	<p id="clipboard-content"></p>

	<hr>

	<button id="copyButton">Set System Clipboard To The Following:</button>
	<p id="para">Ut scelerisque posuere sem, non aliquam ipsum sodales et. Fusce luctus, mauris ut volutpat varius, leo dui posuere ligula, vitae rhoncus leo ipsum eu neque.</p>

	<script src="main.js"></script>
</body>
</html>

## main.js
'use strict';

var extensionId = 'YOUR EXTENSION ID HERE';

document.getElementById('copyButton').addEventListener('click', function() {
	var text = document.getElementById('para').textContent;

	chrome.runtime.sendMessage(
		extensionId,
		{ method: 'setClipboard', value: text },
		function(response) {
			console.log('extension setClipboard response', response);
		}
	);
});

document.addEventListener('DOMContentLoaded', function() {
	readClipboard();

	// Re-read the clipboard every time the page becomes visible.
	document.addEventListener('visibilitychange', function() {
		if (!document.hidden) {
			readClipboard();
		}
	});
});

function readClipboard() {
	chrome.runtime.sendMessage(
		extensionId,
		{ method: 'getClipboard' },
		function(response) {
			document.getElementById('clipboard-content').textContent = response;
			console.log('extension getClipboard response', response);
		}
	);
}

## manifest.json
{
  "manifest_version": 2,
  "name": "Copy clipboard.",
  "description": "Copy clipboard programmatically at any time.",
  "version": "1.0",
  "background": {
    "persistent": false,
    "page": "background.html"
  },
  "permissions": [
    "clipboardRead",
    "clipboardWrite"
  ],
  "externally_connectable": {
    "matches": [
      "http://localhost/*"
    ]
  }
}
	<!DOCTYPE html>
	<html>
	<head>
	<meta charset="utf-8">
	<script src="background.js"></script>
	</head>
	<body>
	<textarea id="ta"></textarea>
	</body>
	</html>
	'use strict';

	function getClipboard() {
	var result = null;
	var textarea = document.getElementById('ta');
	textarea.value = '';
	textarea.select();

	if (document.execCommand('paste')) {
	result = textarea.value;
	} else {
	console.error('failed to get clipboard content');
	}

	textarea.value = '';
	return result;
	}

	function setClipboard(value) {
	var result = false;
	var textarea = document.getElementById('ta');
	textarea.value = value;
	textarea.select();

	if (document.execCommand('copy')) {
	result = true;
	} else {
	console.error('failed to get clipboard content');
	}

	textarea.value = '';
	return result;
	}

	chrome.runtime.onMessageExternal.addListener(function(request, sender, sendResponse) {
	switch (request.method) {
	case 'getClipboard':
	sendResponse(getClipboard());
	break;
	case 'setClipboard':
	sendResponse(setClipboard(request.value));
	break;
	default:
	console.error('Unknown method "%s"', request.method);
	break;
	}
	});
	'use strict';

	var extensionId = 'YOUR EXTENSION ID HERE';

	document.getElementById('copyButton').addEventListener('click', function() {
	var text = document.getElementById('para').textContent;

	chrome.runtime.sendMessage(
	extensionId,
	{ method: 'setClipboard', value: text },
	function(response) {
	console.log('extension setClipboard response', response);
	}
	);
	});

	document.addEventListener('DOMContentLoaded', function() {
	readClipboard();

	// Re-read the clipboard every time the page becomes visible.
	document.addEventListener('visibilitychange', function() {
	if (!document.hidden) {
	readClipboard();
	}
	});
	});

	function readClipboard() {
	chrome.runtime.sendMessage(
	extensionId,
	{ method: 'getClipboard' },
	function(response) {
	document.getElementById('clipboard-content').textContent = response;
	console.log('extension getClipboard response', response);
	}
	);
	}
	{
	"manifest_version": 2,
	"name": "Copy clipboard.",
	"description": "Copy clipboard programmatically at any time.",
	"version": "1.0",
	"background": {
	"persistent": false,
	"page": "background.html"
	},
	"permissions": [
	"clipboardRead",
	"clipboardWrite"
	],
	"externally_connectable": {
	"matches": [
	"http://localhost/*"
	]
	}
	}