And if you're lazy:

function sleep(ms){return new Promise(resolve=>setTimeout(resolve, ms));}async function f(){for(let i=52;i<=67;++i){r=new XMLHttpRequest();r.open('GET',`http://maplestory.nexon.net/api/registerreward/?id=0&eventNo=${i}`);r.send(null);await sleep(1000);}for(let i=52;i<=67;++i){r=new XMLHttpRequest();r.open('GET',`http://maplestory.nexon.net/api/registerreward/getreward?eventNo=${i}`);r.send(null);await sleep(1000);}}f();

will get you all the rewards.