Creating Manual CircleCI Workflow Triggers

CircleCI workflows can be defined in two ways: the regular workflow, where the workflow is triggered on each and every commit, and the scheduled workflow, which is triggered according to the specified schedule.

The problem with workflows is that they cannot be targeted directly for API and web interface triggers! We can only trigger a pipeline for a project branch, and when we do so all the non-scheduled workflows are run.

So how do we configure a workflow that only runs when we trigger it manually?