You may trigger execution on receipt of a webhook from an external source.
Create an IncomingWebhook resource that is configured to use an Incoming Webhook Integration, then include it in the
inputresources block of the step to trigger. Any payload from the webhook will be available in the resource's environment variable
$res_<resource_name>_payload, which can be written to a file. You can then use the read_json utility function to retrieve individual elements from the JSON payload into environment variables.