Hi,

First you need to create a new Promotion Program.

There select the Reward as a Free Product and also select the Product you need to give for free and the quantity as well.

Then while creating a Sales order select the Product with which you want to give free product and also select the free product to the order line

Then just click on the Promotion button in the right bottom

Now you can see an extra promotion product is added in the order line and the total amount is reduced.

Regards