How do I show a report to be requested in a diagram

I'm not sure if I understand your question.

Do you want to add specific SQL code that controls this report running, or just textual information?

In the first case, you can add "Additional SQL script" - available in "Model properties" on right-hand side. It doesn't have any graphical representation on diagram.

In the latter case, you can add a text note.

Does this help? If not, please provide more information about this report running mechanism.