As an user who develops a (complex) web monitoring scenario, I would like a way to manually trigger a web scenario run.

The reason is that when developing a scenario one may set a run frequency of 1 run/minute to ensure "relatively" fast turnaround times, but e.g. leaving for a coffee break or taking longer than a minute may trigger unwanted requests to the server. Additionally, especially when developing against systems with authentication, it may be a good idea for a run to not run automated until development is complete, to avoid automated lock-outs due to authentication failure.