So I got this working by adding a code block on the page where the pop up form appears (thanks Paul2009 but wasn't comfortable sharing my website password yet). Use inspect element in google chrome to get the textarea id.
Inside the code block:
<script>
function limitTextArea(targetText) {
targetText.setAttribute("maxlength",300)
}
// set up the mutation observer
var observer = new MutationObserver(function (mutations, me) {
// `mutations` is an array of mutations that occurred
// `me` is the MutationObserver instance
var targetText = document.getElementById('textarea-yui_3_17_2_1_1594284193900_17624-field');
if (targetText) {
limitTextArea(targetText);
me.disconnect(); // stop observing
return;
}
});
// start observing
observer.observe(document, {
childList: true,
subtree: true
});
</script>
