@chaimpaperman the answer is pretty similar to this post: https://connective.celigo.com/t/create-created-from-linked-work-order-in-netsuite-for-special-work-order-line-item-on-sales-order/276.
You can't link without using the native button. So you will either need to use the native button or create a custom field on the transaction line that is linked to the PO.