In actions_workflows.go, the CreateWorkflowDispatchEventRequest type lists the following documentation of the Ref argument:

// Ref represents the reference of the workflow run.
// The reference can be a branch, tag, or a commit SHA.
// Ref is required when creating a workflow dispatch event.
Ref string `json:"ref"`

However, GitHub's REST API restricts that ref to just a branch or tag name. We initially thought we could build against a specific commit due to this documentation, but further investigation after a failure shows that's not accurate:

ref | string | body | Required. The git reference for the workflow. The reference can be a branch or tag name.