To enable this for a customer you must:
For information on how to enable self-service for your end customers please see this article.
Make sure you've selected your own (partner) organization in the upper right hand drop-down. From the SpinPanel side menu, navigate to Settings > Relationship settings. From the Relationship drop-down select the customer and click SpinPanel settings to open the SpinPanel settings pane. Click the Edit button and make sure the option Auto license adjustment is disabled.
Select the customer organization in the upper right drop-down. From the SpinPanel side menu, navigate to Settings > Role management. Add a role or edit an existing role and make sure the access control Subscription > Subscription quantity is enabled.