This event occurs when someone triggers a workflow run on GitHub or sends a POST request to the “Create a workflow dispatch event” endpoint.
This is saying that when a user clicks the “Run workflow” button on GitHub or executes the Create a workflow dispatch event API, a webhook will be sent to the related GitHub repository to trigger the workflows run on the workflow_dispatch event.
On the repository’s Settings > Webhooks page, it allows users to sent the webhooks for some events to the external services.
Currently, not all the webhooks for the events on GitHub can be sent to the external services. I check the list of the events that we can select, it seems that the Manual events (workflow_dispatch and repository_dispatch) are not allowed to send webhooks to the external services.
If your projects really need to send the webhooks for the Manual events to the external services, as I mentioned in previous reply, you can directly report a feature request here to ask adding the Manual events in the list.