If you need to send traffic to a multicast group, to see if multicast working, you can easily use the ping tool. You have add the „bypass-routing“ option, to make sure the traffic get out without a lookup.
Here is an example:
ping 126.96.36.199 bypass-routing interface ge-0/0/1 count 10000
You can use the „interval 0.1“ option to increase the packets per second.
If you want a receiver you could add a static igmp join, but this only create the forwarding state. If you also need a reply to the multicast ping traffic you must add a listener, like this:
set protocols sap listen 188.8.131.52