Unique identifier linking an entity to the form that created it?

What is the best practice for linking an entity to the form that created it?

I register a participant and save a subset of the data about them to an entity.

I fill out a pre-event survey, selecting their name from a list of entities, and it saves the Entity ID - linking that pre-event survey to the entity.

But I don't see anything comparable available by default between the initial registration survey and the entity?