Simple LED Mod Question

So I have an LED on my Case, and I wanted to attach an extra LED to it for a simple mod. Would I have to connect it in series or in parallel with the other LED, and would I need any extra resistors for that LED? I did electronics wayy back in high school, but I can't remember the formula for working this stuff out. Any help appreciated!

just get the positive on the led and connect to the other positive and same with the ground, watch and see if it dims because if it does you might need a little transistor and resistor