A google for "lemongrass tea recipe" yields this recipe from
Growing and Using Lemongrass
1 tsp. lemongrass
1 cup boiling water
Pour the water over the lemongrass and steep for 5 minutes. Sweeten as desired. Makes one cup of tea.
Now, we don't know how much barkshack you have on hand, but, you could just divide your total amount by 8 oz and you could use this result as a rough guide to the amount of lemongrass to use, of course don't use the same amount of water you would to make a regular tea.
What I would do, is half the amount that this resulted in. For instance, if I had 128 oz of barkshack to flavor I would divide 128/8=16 teaspoons of lemongrass, then I would divide that amount by 2 = 8 teaspoons of lemongrass then I would simmer that for 5 or 10 or 15 minutes or whatever in say 16-24 oz of water, strain, then simmer some more until your satisfied with the reduction of liquid, maybe down to 8 oz. If your going to make a sparkling barkshack now is a good time to add your fermentable for carbing to the lemongrass "extract", pasteurize the fermentable, then put that into your container and rack the barkshack on top of it. Then proceed....