If your Kubernetes API server requires TLS authentication, you must upload a CA Certificate file, when attaching your cluster (This is optional if your cluster does not require TLS authentication from clients).

You can find the CA Certificate, in the certificate-authority-data  key, of your ~/.kube/config file. The value is base64 encoded in this file. The screen above will accept either plain-text, or base64 encoded strings. 

The screenshot below, shows the equivalent value, but is base64 encoded:

If your ~/.kube/config  file does not contain a certificate-authority-data  key, then check for a key named certificate-authority . The value of the certificate-authority key is a path to a local file that contains the certificate. In this case, you would copy and paste the contents of this file into the screen above. 

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