To use the HTTP API, connect your computer to the UPLINK1 network of the gateway and find the gateway's IP address in the Gateway: Info page2 of the webapp.
The HTTP API can be reached on port 4000 of this IP address. Make sure to include the version, v1, in the URL.
From version 2020.3 on it is also possible to reach the api via the default HTTP port, port 80. You will need to include api and the version, v1, in the URL.