OpenCart itself does not ship this API — a plugin is required
Do not look for the REST Admin API key in the OpenCart admin panel; it is not there. OpenCart's own core does not include the admin API this integration needs — it is added by a plugin, and the key comes from that plugin. Scanning the panel end to end and not finding it does not mean you did something wrong; it is simply not there to find.
The second field is the store address, and the API path is derived from it. This is an installation running on your own site; unlike a marketplace there is no fixed service address. If the site address changes, the connection has to be updated too.