To configure Multicast L3 with PIM Sparse Mode when not the rendezvous point:
Go to Network > Virtual Routers and select desired virtual router.
Click Multicast.
Enable Multicast globally by checking the box.
3a. Next, add the Remote Rendenvouz Point by clicking "Add".
Go to Interfaces and click Add.
Add the interfaces to participate in Multicast.
Under Group Permissions, add the multicast groups for which the traffic is permitted.
Make sure to enable IGMP on the interface facing the client.
Make sure PIM is enabled on the interface connecting to the RP.
Configure a security policy to allow the traffic. The destination zone should be 'multicast' and the destination address can be the multicast group addresses.