<button class="js-textareacopybtn">Скопировать</button>
var copyTextareaBtn = document.querySelector('.js-textareacopybtn');

copyTextareaBtn.addEventListener('click', function(event) {
var copyTextarea = document.querySelector('#forumcode');
copyTextarea.focus();
copyTextarea.select();

try {
var successful = document.execCommand('copy');
var msg = successful ? 'successful' : 'unsuccessful';
} catch (err) {
//если нужен, вывод ошибки
}
});