I need HTML BLOCK to add the product additional information as a tab

Hello,

You need to create HTML block: https://xtemos.com/docs-topic/html-blocks-usage/

Then set the Additional tab as the global: one and the same for all the products: Theme Settings > Single Product > Tabs.

If you want to set different tabs for different products, you need to create as many HTML blocks as many products you have and select the HTML block for each product in the single product settings: https://xtemos.com/docs-topic/product-page-options/

Best Regards