Can Authenticate New-CsOnlineSession in PowerShell Command Prompt, but not .NET Core Application

Question

I am using the below code to create a new-csonlinesession in order to add new PSTN gateway.I am trying to setup new session and try to create new pstn usage:

    Import-Module SkypeOnlineConnector
    $secpasswd = ConvertTo-SecureString '******' -AsPlainText -Force
    $userCredential = New-Object System.Management.Automation.PSCredential ('****@****.com', $secpasswd)
    $sfbSession = New-CsOnlineSession -Credential $userCredential
    Import-PSSession $sfbSession
    Set-CsOnlinePstnUsage -Identity global -Usage @{add="International"}

    public static void ExecuteSynchronously()
            {
                using (PowerShell PowerShellInstance = PowerShell.Create())
                {
                    // use "AddScript" to add the contents of a script file to the end of the execution pipeline.
                    // use "AddCommand" to add individual commands/cmdlets to the end of the execution pipeline.
                    // PowerShellInstance.AddScript("param($param1) $d = get-date; $s = 'test string value'; " +
                    //"$d; $s; $param1; get-service");
                   
                    PowerShellInstance.AddScript("Set-ExecutionPolicy RemoteSigned");
                    PowerShellInstance.AddScript("Import-Module 'SkypeOnlineConnector'");
                    PowerShellInstance.AddScript("$secpasswd = ConvertTo-SecureString '*****' -AsPlainText -Force");
                    PowerShellInstance.AddScript("$userCredential = New-Object System.Management.Automation.PSCredential ('****@*****.com', $secpasswd)");
                
                    PowerShellInstance.AddScript("$sfbSession = New-CsOnlineSession -Credential $userCredential");
                    PowerShellInstance.AddScript("Import-PSSession $sfbSession");
                    PowerShellInstance.AddScript("Set-CsOnlinePstnUsage -Identity global -Usage @{add='International - 6'}");
                  
    
                    // invoke execution on the pipeline (collecting output)
                    Collection<PSObject> PSOutput = PowerShellInstance.Invoke();
    
                    // check the other output streams (for example, the error stream)
                    if (PowerShellInstance.Streams.Error.Count > 0)
                    {
                        // error records were written to the error stream.
                        // do something with the items found.
                    }
    
                    // loop through each output object item
                    foreach (PSObject outputItem in PSOutput)
                    {
                        // if null object was dumped to the pipeline during the script then a null
                        // object may be present here. check for null to prevent potential NRE.
                        if (outputItem != null)
                        {
                            //TODO: do something with the output item 
                            Console.WriteLine(outputItem.BaseObject.GetType().FullName);
                            Console.WriteLine(outputItem.BaseObject.ToString() + "\n");
                        }
                }
            }
        }

Powershell code is working as expected.But getting error in c#.
I am getting "System.Management.Automation" "Could not find WWW-Authenticate header in response" error ?