Support multiline

I suggest to change input element to textarea element.And add newline seperator to split function.

Split:

keys = input.trim().split(/[\s,]+/);

Change element type:

function changeElementType(element, newtype) {var newelement = document.createElement(newtype);while(element.firstChild) newelement.appendChild(element.firstChild);for( var i=0, a=element.attributes, l=a.length; i<l; i++) {newelement.setAttribute(a[i].name, a[i].value);}element.parentNode.replaceChild(newelement, element);return newelement;}var eleInput = document.getElementById('product_key');eleInput = changeElementType(eleInput, "textarea");

I test this work by modifying your script.