function processForm() {
var listItem = document.myCheckListForm.checklistItem.value;
var listed = '<div><input type="checkbox" />' + listItem + '</div>';
document.getElementById("checkList").innerHTML += listed;
}
<html>
<head>
<script type="text/javascript" src="script.js"></script>
</head>
<body>
<form name="myCheckListForm">
<input type="text" name="checklistItem">
<input type="button" value="Add to List" onclick="processForm()">
</form>
<div id="checkList"></div>
</body>
</html>