Forms - Changing the label of a text field on a selection

Hi everyone,

We're trying to change the label of a text field on a dropdown selection. Based on the same site (pw: "new-website"), what we're tryign to do is:

1. When the Dropdown is "One", Change the label "Message" to "Message One"

2. When the Dropdown is "Two", Change the label "Message" to "Message Two"

I've devised the following code, which when inputting into the console, does manage to change the field, but when placed within the custom footer it doesn't work.

<script>  
  messageobject = document.getElementById("textarea-yui_3_17_2_1_1707503296891_717");
  messagelabel = messageobject.getElementsByTagName("span")[0];
  messagelabel.innerHTML = "TESTTESTTEST";
</script>

Any help would be appreciated!