How do I leave the NTEU Union?

Hey everybody, I’m a member of NTEU CHAPTER 71 Union and I am looking to leave (I am probationary and signed the fork).

Could anyone give some insight on how to leave the Union? I’m under the impression I need to fill out the sf-1188 but I’m stumped on the 4th question - what my “timekeeper number” is.

Anyone know where I can find this information? Or have any insight on how to leave? Thanks a lot