It is not obvious now how to use this to create a new playlist that connects to an existing rdm session.
It appears it wants me to pass in a object of type Devolutions.RemoteDesktopManager.Business.Connection which I don't know how to construct and assuming I figure that out it doesn't look like it accepts a Group parameter so I can specific which folder to place it in.
fyi, I found a workaround using new-rdmsession. here is the code in case it helps someone:
$cpl=new-object -TypeName Devolutions.RemoteDesktopManager.Business.ConnectionPlayListItem
$s=New-RDMSession -type playlist -name $n -Group $g
Set-RDMSession -Session $s