bibliotheca-php/webclient/view/default/manageentry/field-text3.html

<div class="uk-margin">
	<label class="uk-form-label" for="<?php echo $field['identifier']; ?>"><?php echo $I18n->t($field['displayname']); ?></label>
	<div class="uk-form-controls">
		<textarea class="uk-textarea" autocomplete="off"
				  id="<?php echo $field['identifier']; ?>"
				  rows="3" maxlength="255"
				  name="fdata[<?php echo $field['identifier']; ?>]"><?php echo $TemplateData['editData'][$field['identifier']] ?? ''; ?></textarea>
		<small id="<?php echo $field['identifier']; ?>_counter" class="uk-float-right"><?php echo 255-strlen($TemplateData['editData'][$field['identifier']] ??  ''); ?></small>
		<?php if(isset($field['bulkedit'])) { ?>
		<select class="uk-select" name="fdata[additionalEditOption][<?php echo $field['identifier']; ?>]">
			<option value=""><?php echo $I18n->t('manageentry.select.bulkedit'); ?></option>
			<option value="add"><?php echo $I18n->t('manageentry.select.bulkedit.add'); ?></option>
			<option value="replace"><?php echo $I18n->t('manageentry.select.bulkedit.replace'); ?></option>
			<option value="empty"><?php echo $I18n->t('manageentry.select.bulkedit.clear'); ?></option>
		</select>
		<?php } ?>
	</div>
</div>

<script type="text/javascript">
	{
	const textarea = document.getElementById("<?php echo $field['identifier']; ?>");
	const textareaCounter = document.getElementById("<?php echo $field['identifier']; ?>_counter");

	textarea.addEventListener("input", event => {
		const target = event.currentTarget;
		const maxLength = target.getAttribute("maxlength");
		const currentLength = target.value.length;

		textareaCounter.innerHTML = `${maxLength - currentLength}`;
	});
	}
</script>