I was wondering which symbol should be used for representing a user event (i.e. button click).
Since this question remains unanswered, I'd like to expand on my comments.
In a flow chart, you probably don't need to represent events at the level of detail that you are thinking. A flow chart is a representation of a workflow, process, or algorithm. At the most basic level, it contains terminals (a start condition or event and one or more stop conditions or events), processes that cause data to change, decisions that need to be made, and input and output. There are some other concepts, such as referring to other processes (which may be documented in other flow charts), representing documents rather than just data, or manual operations that exist outside of the workflow or algorithm.
If you are modeling software, a button click is a very detailed event. If anything, the button click is most likely to be a terminal - the act of clicking a button starts a process. However, I wouldn't refer to this as a button click in a flow chart. This button click represents something at the process level - submitting data or forms, perhaps. That would be the true starting terminal. Alternatively, the entire process of a user completing a form could be represented as input - this would include every keystroke needed to complete the form as well as a button click or keystroke to submit it.
When you are creating flow charts, abstract away the user interface. Even abstract away the software. Think in terms of the data and information that you need and what high level events start the process. Put those onto the flow chart.