I doubt that bees will be drawn by lemongrass plants nearby. It is the internal sap of the leaves infused into oil that is potent and effective.
I take a Qtip, dip it into some good quality lemongrass oil. Dab at the box entrance, mark the top of one of the frames, place the Qtip in a small zip lock bag and put it into the bottom of your swarm box.
And as Ben Framed said, a previously used box or frames gives it an irresistible bee smell they like.