That's a stem plant -- it'll get almost all of its nutrients from the water, not the substrate (gravel). With a plant like this, it's generally suggested you take it out of the pot and carefully remove the rockwool from around the roots (edit: unless it's the kind of stem plant with roots that can become invasive -- in which case some people prefer to leave them potted -- but I don't think that's the case with Limnophila).
Then, just 'plant' it, basically. Nothing special needed, just bury the root end under enough gravel to keep the plant down.