If you don’t have access to the markup for the input field, you can make it read-only via jQuery.
Make an input field read-only with jQuery
JavaScript
// jQuery
$(document).ready(function() {
$('#fieldID').attr('readonly', 'readonly'); // Replace #fieldID with your input field's ID
});
Paste this snippet into a JavaScript file that is loaded by your page, or you can put it between <script> </script>
tags in the footer.
jQuery is required.