For pritunl-link the access key would be set on the pritunl-link client using the command shown in that documentation. The key in the Pritunl web console is for the route advertisements on VPN servers. The Pritunl server does not function as a link client. Every link must have at least two locations and one host in each location. The pritunl-link client should not be run on the same instance as a Pritunl server.