You can use the failover command set to enable a cluster.
On primary unit:
failover lan unit primary
failover lan interface faillink Vlan20
failover key 1234
failover link statelink Vlan21
failover interface ip faillink 1.1.1.1 255.255.255.252 standby 1.1.1.2
failover interface ip statelink 1.1.1.5 255.255.255.252 standby 1.1.1.6
failover
On secondary unit:
failover lan unit secondary
failover lan interface faillink Vlan20
failover key 1234
failover interface ip faillink 1.1.1.1 255.255.255.252 standby 1.1.1.2
failover