Percona Xtradb Cluster does not accept options on the command line

I am using Percona Xtradb Cluster 5.6 and try to start my cluster through a donor node through the command line and this error appears:

CentOS 7:

systemctl start mysql –wsrep_sst_donor = srv1

systemctl: unrecognized option & # 39; – wsrep_sst_donor = srv1 & # 39;