In order to hook up a powered subwoofer properly, an RCA "splitter" cable ("Y" cable) is not needed.
Any single, standard RCA or subwoofer cable should do just fine. However, if the sub is exhibiting a hum with the RCA/sub cable plugged into it, then maybe try using a "shielded" subwoofer cable.
Monoprice, for example, is a manufacturer of quality shielded subwoofer cables: https://www.monoprice.com/product?p_id=2684
One end of the subwoofer/RCA cable will be plugged into the Subwoofer Output on the back of your receiver. This output jack is sometimes labeled as SUB, SUB OUT, SUB PREOUT etc.. The other end of the cable will be plugged into the L/LFE (white) RCA jack on the back of your sub.
Any cable designed for subwoofer use should be just fine. It doesn't have to be expensive; the cheaper ones work just fine as well. However, if you notice the subwoofer humming excessively, then you may want to try a "shielded" subwoofer cable.
For more information on this "humming" and how to prevent it, please see our article - Thumping/Humming Noise in Sub
In addition, if you still use the RCA "Y" splitter cable, you have the potential to boost the sub's output signal by 3dB when plugging into both the red and white RCA inputs on the back of the sub.
For more information on the best way to hook up a subwoofer, please click here to view.