Decoding accelorometer by tektelic

I defined payload codec. By defining a payload codec, Application Server can encode and decode the
binary device payload for you. However TekTelic tells me i have to use JSON DC that is pasting JS code. I can not find where tom paste the code in chirpstack. I only find how to define payload codec. Is there a place to paste JS code to generate JSON DC? if someone has JS code how to decode output of accelorometer by TekTelic?