REST discovered
https://connect.signl4.com/api
X-S4-Api-Key: <credential>
In the SIGNL4 web portal, go to Teams -> Developer -> API Keys and create an API key. The docs state that the REST API requires this key and that you send it in the X-S4-Api-Key header on each request. See Meta API and REST API.
Teams -> Developer -> API Keys
X-S4-Api-Key