Uses openid /.well-known on Entra to look for a Tenant ID

Get-TenantIdFromDomainName.ps1

function Get-TenantIdFromDomainName {
    param (
        [Parameter(Mandatory = $true)]
        [ValidatePattern('^([a-zA-Z0-9-]+\.)+[a-zA-Z]{2,}$')]
        [string]$DomainName
    )

    try {
        # Construct the URL
        $domLookupUrl = "https://login.microsoftonline.com/$DomainName/.well-known/openid-configuration"
        
        # Fetch the configuration
        $response = Invoke-RestMethod -Uri $domLookupUrl -Method Get -ErrorAction SilentlyContinue
        
        # Parse the tenant ID from the authorization endpoint
        $authorizationEndpoint = $response.authorization_endpoint
        if ($authorizationEndpoint -match '/([a-f0-9-]{36})/') {
            $tenantId = $matches[1]
            return $tenantId
        } else {
            throw "Tenant ID not found in the authorization endpoint."
        }
    } catch {
        Write-Warning "An error occurred: $_"
        return $null
    }
}

#Example Usage
$domainName = Read-Host -Prompt "Enter Domain Name"

Write-Progress -Activity "Extracting Tenant ID" -Status "Please wait..." -PercentComplete 50;
$tenantId = Get-TenantIdFromDomainName -DomainName $domainName;
if($tenantId) {
    Write-Progress -Activity "Extracting Tenant ID" -Status "Tenant ID extracted successfully." -Completed;
    Write-Host "Tenant ID extracted from domain name: $tenantId" -ForegroundColor Green;
} else {
    Write-Progress -Activity "Extracting Tenant ID" -Status "Failed to extract Tenant ID." -Completed;
    Write-Host "Failed to extract Tenant ID from domain name $domainName. There may be no tenant at this domain, verify your domain name. Exiting script." -ForegroundColor Red;
    pause;
    exit;
}