What must you select on a purchase form when using a product or service?

Selecting "I purchase this product/service from a vendor" on a purchase form is crucial because it establishes the source of the product or service being acquired. This option confirms that the transaction is being made through a vendor, which is essential for recording accurate financial information and maintaining the integrity of the accounting records. It ensures that the system understands this purchase is linked to vendor transactions, thereby affecting accounts payable and inventory management appropriately.

The other options do not address the basic requirement of documenting where the product or service is sourced. For instance, adding a new customer is relevant only when creating customer records and is not necessary for a purchase transaction. Similarly, indicating discounts or simply adding to an invoice does not provide the foundational link to vendor purchases, which is essential for effective bookkeeping and financial reporting. Therefore, the correct choice emphasizes the importance of correctly identifying and documenting the vendor in the purchasing process.