Separately create a product, then attach to a form.
Navigate to Products page via side menu, Order Forms > Products.
Click 'Add New Product'.
Fill up the product detail and click 'Add'.
Firstly choose your form that you want to edit.
Go to Form Edit / Product.
Inside a section you may see a button called 'Add Product Ref'
Select the items from the dropdown menu.
You may select multiple items as well. Once you selected the items, click 'Add'.