Textarea “admin-note” as required

Hi, nice plugin.

How I can make Textarea “admin-note” – required field?

<textarea id="<?php echo $this->id; ?>-admin-note" class="input-text" type="text" name="<?php echo $this->id; ?>-admin-note"></textarea>

I try insert “required”

<textarea id="<?php echo $this->id; ?>-admin-note" class="input-text" type="text" name="<?php echo $this->id; ?>-admin-note" required></textarea>

but customer buy goods without fill textarea