That depends on what you want to achieve.
the source of a list can be created objects added to a list, a result of a retrieve activity in a microflow, can be be the result of a query triggered from a list widget, it can be the input parameter of a microflow…….
As you see, there are a few options. Question for you; what do you want to achieve from a functional perspective?
The process is key to your solution.
So you need a microflow which executes 1 when a user hits a button. My guestimation is that the user selects an event and based on that selection the registration is created. Thus the microflow can start with an input parameter of type Event
Microflow would look like this:
Answering your original question; The list need to be retrieved from the database ;-)