Please contact your Account manager and request for a new temporary license. It should have the key to enable JSS. If you are a partner, the new partner license should also have that key. That should be sufficient to do some prototyping. For development and product, you would need to talk to your Account manager regarding conversion to subscription license. If you are not sure who you Account rep is, please email jss@sitecore.net, we will find out.
Cheers!