Here is my example from Powershell that works fine when I use something like admin@local as username:
$auth = $username + ':' + $password
$Encoded = [System.Text.Encoding]::UTF8.GetBytes($auth)
$EncodedPassword = [System.Convert]::ToBase64String($Encoded)
$headers = @{
"Accept"="application/json"
"Authorization"="Basic $($EncodedPassword)"
"Content-Type"="application/json"
}
$body = ""
$jsonbody = $body | ConvertTo-Json
$method = "POST"
$api = "/lcm/authzn/api/login"
$res = $null
$res = Invoke-WebRequest -Uri ($server+$api) -Method $method -Body $jsonbody -Headers $headers -UseBasicParsing -SessionVariable session
$res.StatusCode
If ($res.StatusCode -ne 200) {"ERROR!"; BREAK}