Go to Income > Invoice, and click on add New Invoice
1) Fill in the details like customers, invoice date, place of supply, etc.
2) Add the products by selecting the product from items drop-down menu
3) Add quantity on invoice and mark the price as zero, this will ensure inventory is adjusted and goods are offered free on invoice. You can enter the details in description box.
4) Click on continue, enter the warehouse details if available.
5) Click on continue, your free product will be added to invoice.
Refer the below mentioned image for better reference:
Alternatively, you can enter price for the specific product and enter the discount at 100% like the image shown below: