Hi @jspft
I understand that you would like to have a custom button on the order edit screen (when creating manual orders), which adds a specific product to each order, without you having to manually add this product.
I wasn’t able to find any plugins that does this, unfortunately. There are also no readily available code snippets that would add such button to the order page.
There are however, code-based ways to programatically create orders, and then add product x to these orders. There are also ways to add product x to an order programatically when an order is created and a status changed, such as https://stackoverflow.com/questions/65355968/add-a-free-product-programmatically-on-woocommerce-specific-order-status-change (manually create an order and when status is processing, add product x)
Alternatively, considering that there is a lack of such plugins/code, you may want to look into having a developer whip up a customized solution for you.
Cheers!