Jack,
You need to put session token in some storage.
For this goal:
1 Open storage and mode inteface. 
 2 Add storage "sessionToken" with type "string". 
 3 Then open login service and click "Success" mapping. 
 4 Make link from session token response parameter to "sessionToken" storage. 
 5 From this time you can link actual session token from this storage.
Regards.