how to import child records

I'm posting this even though I found some info by searching the forum. That info was from many years ago prior to v9.7 when session variables were changed... so, those fixes were not working.
I'm trying to import students.
I have two tables.
rosters
students
the info I found in the forum said to use the following code in Import before Insert Record
$values['roster_ID'] = $_SESSION["<table name>_masterkey1"];
when I change that to
$values['roster_ID'] = $_SESSION["rosters_masterkey1"];
it does not work. It is inserting blank for that field.
I can see the value I need in the URL https://www.mywebsite.com/class/student_list.php?mastertable=rosters&masterkey1=5767&a=return