As per the HP - ALM REST documentation, you should be calling the below method with Basic credentials in order to get the LW-SSO token,
GET /qcbin/authentication-point/authenticate
You don't need to pass Basic credentials to the method GET /qcbin/rest/is-authenticated at all, since its is just a validator to check whether you are authenticated and it is an optional step too. I was successfully able to signin to ALM through this process.
Thanks,
Samy
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