The recommended workflow when purchasing products in WorkGuru for the first time is add or import the products to the WorkGuru products database first, then add them to your purchase order.
However, WorkGuru allows you to add ad-hoc purchase lines to the database retrospectively. WorkGuru defines an ad-hoc product line as one where it's SKU does not existing in the product database.
When raising a new PO, add your new product as an ad-hoc line with as much detail as you can. Ensure you have added an SKU. You cannot add ad-hoc lines to your product database without an SKU.
WorkGuru identifies ad-hoc purchase lines with an asterisk. Approve the purchase order as per the normal workflow. When receiving the purchase order, WorkGuru will detect ad-hoc lines are present and ask you if you want these created in the products database. 
The product is then added to the database once the purchase order is received. NOTE - the product will be added to the product database as a NonStock product by default. If the ad-hoc product purchased needs to be added to stock on hand, it will need to be moved in via a stock adjustment.
The product will be setup with as much or as little information you provided in the adhoc line on the purchase order. Edit the product to add additional product details and to switch to FIFO etc as required.