Ah, that tripped me up when I started with Blueprints.
What you need to do is first get a reference to the blueprint instance in your level. To do this, select the blueprint in the level, go into your level blueprint and right click and choose “Add reference to [your instance name here]”
This will give you a node. Drag the blue line off this node and you will be able to call the LaunchShell event!
This is a common workflow in Blueprints: You will need a reference to a certain class of object to access the methods, events and variables that are specific to that class.