Hi Pascal,
Have you also read the doc too?
https://docs.microsoft.com/en-us/dynamics365/marketing/score-manage-leads#set-up-a-condition
For example, you could add an expression for "City = New York", which would modify the condition so that only email clicks made by contacts in New York City would trigger the condition.

As the advanced lead scoring conditions said:
https://docs.microsoft.com/en-us/dynamics365/marketing/score-manage-leads#create-advanced-lead-scoring-conditions-by-using-traversals
You can also build scroing criteria based on event registration related contact field data by adding a dot after the entity:

In your scenario, I thought there is no need to add an expression if you would just give points to someone when an event registration record is created from him/her.
For example, if I set Event registration contains some value as expression,

As what we see in Text preview mode:

The expression stands for event registration id, however, the id of an event reigstration record is a random guid serial numbers,
which makes no sense for us to do a filter for records based on this.
So you could append dot after the entity if you would like to score related people with specific criteria.
Here is another detailed tutorial about how to design lead scoring model:
https://www.preact.co.uk/blog/microsoft-dynamics-365-for-marketing-automatic-lead-scoring
Regards,
Clofly