It looks like some legacy HTML tags are needed. Try adding the "name" parameters to your api_key and field4 input box.
<form action="https://api.thingspeak.com/update.json" method="post"> <p> <label for="set temp">Desired Temperature: </label> <input type="hidden" id="api_key" name="api_key" value="XXX"> <input type="text" id="field4" name="field4"> <br> <input type="submit" value="send"> </p> </form>