Since these tricks might be forbidden by official website, I just give a summary here and would complete it in the future.
- Login in you JLPT account in the browser.
- Try “Inspect” by right click the grayed button “Book” and you will find the HTML and Javascript code.
- Run this code in the console.
Of course, you cannot book tickets before the system is open, but it would make you to fill the trouble check code before, which is extremely useful for many people. Actually, I booked tickets successfully in this way every time.