CUSTOM Trunk dial string to dial Ring Group or Local Extension

Hello Friends,
Hope you are doing well. I am facing a problem and I hope that my expert friends can help me to recover it. I've setup a ring group and I want to setup CUSTOM Trunk's destination to ring group. But I don't know what Custom Dial String will be. For example, to dial an IAX2 trunk, I use "IAX2/MyIAX/$OUTNUM$" in the custom dial string of custom trunk.
Anybody can tell me what will be the custom dial string to dial a ring group extension (ring group 600).
Please help.
thank you.