npm install -g snarkjs
# or
yarn global install snarkjs

# first get the latest contribution
$ ls -1 ./participations | tail -n 1
4201_anoynmous.zkey

# do you contribution
# Feel free to put your name / twitter handler / keybase
$ snarkjs zkey contribute ./participations/00xx_anoynmous.zkey ./participations/4202_my_name.zkey --name="My Name" -v

$ wget https://hermez.s3-eu-west-1.amazonaws.com/powersOfTau28_hez_final_16.ptau 
$ snarkjs zkey verify ./build/withdraw.r1cs ./powersOfTau28_hez_final_16.ptau "./participations/4202_my_name.zkey"