How do I add a dropdown field in the commerce form? Does someone know how to add a dropdown field to the commerce form?

I need to add a field where people need to select one of the 3 options. Ex.: "How did you know us?" (options: Google, a friend, I don't know)

Can anyone help me?

Thanks
I think this is what you are looking for:
/admin/commerce/config/line-items/product/fields

This is where you can add fields to the product line item type.  Make sure you have the Line Item UI module enabled or you might not have access to that UI.