Hi, Dale!
Patrick, a Shopify Guru here :).
At the moment, you wouldn't be able to run a BOGOF promotion using a discount code but you could use an app like Free Gifts or Product Discounts to help run your special offer. :)
Free Gifts has a 17-day trial so you can test is out, after that it's $29.99/month, the Product discount doesn't have a free trial period but it is a once off payment of $99.99.
Hope that helps, feel free to contact us at support@shopify.com if you have any other questions.
Patrick :)