You are using Simple Survey, and would like to send a survey based upon a Task/Event being created or updated.
While you can technically send a survey based upon a Task/Event being created or updated, you can not establish a lookup relationship between the Survey object and Task/Event. This is a limitation of the Salesforce platform.
The importance of establishing the lookup relationship is so that the Survey will be related back to the record from which is was sent.
To workaround this limitation, we recommend updating a field on the parent record (such as Contact / Account) when a Task/Event is created or updated. From there, you can send your survey from the parent record based upon that specific field being updated.
Note: The Task / Event objects are intentionally not exposed in the Survey Builder.