Good news is way back machine had a copy of it: https://web.archive.org/web/20200808154120/https://community.shopify.com/c/shopify-design/adding-a-tag-to-a-customer-through-a-form/m-p/590698
Just add the code below right after {% form ‘create_customer’ %}
<input type="hidden" name="customer[tags]" value="your_tag">