ArcSet has given you an succinct answer, for the node level you say you are trying to reference, but FYI, this is all about basic parsing XML using PowerShell.
PS has XML cmdlets ...
Get-Command -Name '*xml*' | Format-Table -AutoSize
CommandType Name Version Source
----------- ---- ------- ------
...
Cmdlet ConvertTo-Xml 3.1.0.0 Microsoft.PowerShell.Utility
Cmdlet Convert-XMLtoJSON 5.0.0.1 Sorlov.PowerShell
Cmdlet Export-Clixml 3.1.0.0 Microsoft.PowerShell.Utility
Cmdlet Import-Clixml 3.1.0.0 Microsoft.PowerShell.Utility
Cmdlet Merge-XMLFile 5.0.0.1 Sorlov.PowerShell
Cmdlet New-XSDfromXML 5.0.0.1 Sorlov.PowerShell
Cmdlet Select-Xml 3.1.0.0 Microsoft.PowerShell.Utility
... specifically for this or you can use .NET xml namespace to parse it. There is a ton of docs, eBooks and videos on this topic. XML and JSON are a very big deal in PS.
PowerShell Simply Put: Parsing Through XML
Mastering everyday XML tasks in PowerShell
For example, using the .Net xml namespace, change this ...
$XMLObject = Invoke-RestMethod -Method 'POST' -Uri $url -Headers $header -Body $body
... to say this...
[xml]$XMLObject = Invoke-RestMethod -Method 'POST' -Uri $url -Headers $header -Body $body
... Then parse as needed.
$XMLObject.Object
For Example (since there is no way for me to use what you actually have):
# download currency exchange rates in XML format and parse for currency rates:
$url = 'http://www.ecb.europa.eu/stats/eurofxref/eurofxref-daily.xml'
[xml]$result = (Invoke-WebRequest -Uri $url -UseBasicParsing).Content
$result
xml Envelope
--- --------
version="1.0" encoding="UTF-8" Envelope
$result.Envelope
gesmes : http://www.gesmes.org/xml/2002-08-01
xmlns : http://www.ecb.int/vocabulary/2002-08-01/eurofxref
subject : Reference rates
Sender : Sender
Cube : Cube
$result.Envelope.Cube
Cube
----
Cube
$result.Envelope.Cube.Cube
time Cube
---- ----
2018-09-28 {Cube, Cube, ...
$result.Envelope.Cube.Cube.Cube
currency rate
-------- ----
USD 1.1576
JPY 131.23
BGN 1.9558
...