Allow SPO CSOM and Microsoft.SharePoint.PowerShell PSSnapIn to Coexist in the same AppDomain

Add-FormsAuthRequestHeader.ps1

Add-PSSnapin -Name Microsoft.SharePoint.PowerShell

# prevent issues with the SharePoint API certificate validation procedures (must be after loading sharepoint snapin)
[System.Net.ServicePointManager]::ServerCertificateValidationCallback = {$true}

function Add-FormsAuthRequestHeader
{
    param
    (
        [Parameter(Mandatory=$true)][Object]$ClientContext
    )

    begin
    {
        # try to dynamically locate the the CSOM assemblies
        $referencedAssemblies = $([AppDomain]::CurrentDomain.GetAssemblies() | ? { -not $_.GlobalAssemblyCache -and $_.FullName -match "Microsoft.SharePoint.Client." }).Location
    }
    process
    {
        Add-Type -ReferencedAssemblies $referencedAssemblies -TypeDefinition @"

            using System;
            using Microsoft.SharePoint.Client;
   
            namespace WebRequestExecutor
            {
                public static class AuthHeaderHelper
                {
                    public static void AddAuthHeader(ClientContext context)
                    {
                        context.ExecutingWebRequest += new EventHandler<WebRequestEventArgs>(AddAuthHeader);
                    }

                    private static void AddAuthHeader(object sender, WebRequestEventArgs e)
                    {
                        e.WebRequestExecutor.RequestHeaders.Remove("X-FORMS_BASED_AUTH_ACCEPTED");
                        e.WebRequestExecutor.RequestHeaders.Add("X-FORMS_BASED_AUTH_ACCEPTED", "f");
                    }
                }
            }
"@ 
        [WebRequestExecutor.AuthHeaderHelper]::AddAuthHeader($ClientContext);
    }
    end
    {
    }
}


Add-Type -Path "E:\Microsoft.SharePointOnline.CSOM.16.1.6008.1200\lib\net45\Microsoft.SharePoint.Client.dll"
Add-Type -Path "E:\Microsoft.SharePointOnline.CSOM.16.1.6008.1200\lib\net45\Microsoft.SharePoint.Client.Runtime.dll"

$clientContext = New-Object Microsoft.SharePoint.Client.ClientContext( "https://contoso.sharepoint.com/sites/teamsite" )
$clientContext.Credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials( "admin@contoso.onmicrosoft.com", $(ConvertTo-SecureString -String "pass@word1" -AsPlainText -Force ))

Add-FormsAuthRequestHeader -ClientContext $clientContext

$clientContext.ExecuteQuery()