Hi Daniel,
The assumption log is first created in the Initiating group (PMP Exam Outline, Pg 5, Task 4)
Although it is highly unlikely that the exam question will ask what are the inputs for an assumption log specifically, I think you can develop the high-level assumptions based on the business case/agreements/EEFs/OPAs. In later iterations, you can develop your assumptions from other project plans and documents.
The assumption log can also help to identify risks because: "Assumptions and constraints recorded in the assumption log may give rise to individual project risks and may also influence the level of overall project risk." (Pg 412, PMBOK 6)
Hope this helps