SOAP request

The script below doesn't work. Error is "Method invocation failed because [System.String] does not contain a method named 'Save'."

Please advise.

$SOAPRequest = @"
<soapenv:Envelope xmlns:soapenv="">schemas.xmlsoap.org/.../" xmlns:even="">www.abbvie.com/.../EventOccurrenceEBM">
<soapenv:Header/>
<soapenv:building>
<even:EBM>
<even:HeaderMsg>
<even:SourceSystemName>TM1</even:SourceSystemName><even:SystemEnvironmentCode>DEV</even:SystemEnvironmentCode></even:HeaderMsg>
<even:BodyMsg>
<even:EventOccurrenceEBO>
<even:EventDefinition>
<even:EventDefinitionName>TM1 Process Alerts</even:EventDefinitionName>
</even:EventDefinition>
<even:EventOccurrenceDate>2018-02-20T15:10:00.00</even:EventOccurrenceDate>
<even:EventOccurrenceSignature>APP_NAME=HQPS^+^PROC_NAME=save data</even:EventOccurrenceSignature>
<even:EventOccurrenceData>APP_NAME=HQPS^+^PROC_NAME=save data</even:EventOccurrenceData>
<even:NotificationUsers>
<even:NotificationUser>
</even:NotificationUser>
</even:NotificationUsers>
<even:ABORecordInfo>
</even:ABORecordInfo>
</even:EventOccurrenceEBO>
</even:BodyMsg>
</even:EBM>
</soapenv:building>
</soapenv:Envelope>
"@
[String] $URL = 'vtibdvabv.abbvienet.com:11116/.../ABV.RR.HTTP.EventNotificationService_client_ep'

#Try
# {
# Sending SOAP Request To Server
$soapWebRequest = [System.Net.WebRequest]::Create($URL)
# $soapWebRequest.ClientCertificates.Add($cert)
$soapWebRequest.Headers.Add("SOAPAction","notify")
$soapWebRequest.ContentType = "text/xml;charset=UTF-8"
$soapWebRequest.Accept = "text/xml"
$soapWebRequest.Method = "POST"
# $soapWebRequest.Proxy.Credentials = [System.Net.CredentialCache]::DefaultNetworkCredentials
# $soapWebRequest.UseDefaultCredentials = $true

#Initiating Send
write-host "Initiating Send."
$requestStream = $soapWebRequest.GetRequestStream()
write-host "before save."
Write-Host $soapWebRequest
write-host $requestStream
$SOAPRequest.Save($requestStream)
write-host "after save."
$requestStream.Close()

#Send Complete, Waiting For Response.
write-host "Send Complete."
$resp = $soapWebRequest.GetResponse()
$responseStream = $resp.GetResponseStream()
$soapReader = [System.IO.StreamReader]($responseStream)
$ReturnXml = [Xml] $soapReader.ReadToEnd()
$responseStream.Close()
# }
# Catch
# {
# Throw $ReturnXml.Envelope.InnerText

# }

$Return = $ReturnXml.Envelope.InnerText
$SOAPRequest = $SOAPRequest.OuterXML