Add field to Product Details in Sigma

Add field to Product Details in Sigma

For the Quote, Sales Order, and Invoice, there is a section called Product Details in the API. I have successfully added a custom field to main product, but I am struggling with how to add a custom field to the product details section, as it doesn't appear to be an option under the modules.

Would someone be able to either give me an example of how to do this, or point me at the resources that show how to do this?