How can you "write" someone's heart? That's not really a writable thing. You can write a name, or a sentence, or a termpaper...but not a heart. That's crazy! Furthermore, why would you wanna write it on your hand, of all places? Or...maybe she's referring to something else with the word "it", but from this passage, the antecedent would be unknown (and whatever that antecedent is, *could* be written on the hand, but it's unclear for now). Still, it sounds like she's saying 'write your heart on my hand', which clearly doesn't make any sense.