How to make a input field readonly with JavaScript?

I know you can add readonly="readonly" to an input field so its not editable. But I need to use javascript to target the id of the input and make it readonly as I do not have access to the form code (it's generated via marketing software)

I don't want to disable the input as the data should be collected on submit.

Here is the page I have added in the below suggestion with no luck so far:

https://www.pages05.net/engagedigital/inputReadOnly/test?spMailingID=6608614&spUserID=MTI5MDk4NjkzMTMS1&spJobID=Nzk4MTY3MDMS1&spReportId=Nzk4MTY3MDMS1

Make sure you use <body onload="onLoadBody();"> for anyone using this in the future.

Solution

You can get the input element and then set its readOnly property to true as follows:

document.getElementById('InputFieldID').readOnly = true;

Specifically, this is what you want:

<script type="text/javascript">
  function onLoadBody() {
    document.getElementById('control_EMAIL').readOnly = true;
  } 
</script>

Call this onLoadBody() function on body tag like:

<body onload="onLoadBody">

View Demo: jsfiddle.