Search for a certain string or value in a session variable, modify it, and create a new session variable

this should work, use it in a variable assign to whatever session variable you want.

set mem_fields [split [mcget {session.ad.last.attr.memberOf}] "|"]; foreach mem $mem_fields { if { $mem contains "Container" } { set found $mem } }; set grp_fields [split $found "-"]; return [lindex $grp_fields 2];

if not please check apm log for error or what result you do get.