getsiteandwebidfromsiteurl.ps1

#Add references to SharePoint client assemblies and authenticate to Office 365 site - required for CSOM
Add-Type -Path “C:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.dll”
Add-Type -Path “C:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.Runtime.dll”
Add-Type -Path “C:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.WorkflowServices.dll”

#Specify tenant admin and site URL
$SiteUrl = "mysiteurl"
$UserName = "myusername"
$SecurePassword = ConvertTo-SecureString "mypassword" -AsPlainText -Force

#Bind to site collection
$ClientContext = New-Object Microsoft.SharePoint.Client.ClientContext($SiteUrl)
$Credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($UserName, $SecurePassword)
$ClientContext.Credentials = $Credentials
$ClientContext.ExecuteQuery()

# Get WorkflowServicesManager
$WorkflowServicesManager = New-Object Microsoft.SharePoint.Client.WorkflowServices.WorkflowServicesManager($ClientContext, $ClientContext.Web)
$ClientContext.Load($WorkflowServicesManager)
$ClientContext.ExecuteQuery()

# Get Site and Web Id
$ClientContext.Load($ClientContext.Web)
$ClientContext.Load($ClientContext.Site)
$ClientContext.ExecuteQuery()
$ClientContext.Web.Id
$ClientContext.Site.Id