javascript_pupil,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
</head>
<body>
<form action="http://" name="test" id="test">
<input name="bumbum" type="radio" value="5">
<input name="bumbum" type="radio" value="15" checked="checked">
</form>
<script>
test.insertAdjacentHTML('beforeEnd','<input type="text" name="field_name" value="" onChange="alert([this.value, document.querySelector(\'[name=bumbum]:checked\').value])" >');
</script>
</body>
</html>