PS + CSOM to import search schema XML (e.g. managed property definitions)..

ImportSearchSchema.ps1

. .\TopOfScript.ps1

# need some extra types bringing in for this script..
Add-Type -Path "c:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.Search.dll"

# TODO: replace this path with yours..
$pathToSearchSchemaXmlFile = "C:\COB\Cloud\PS_CSOM\XML\COB_TenantSearchConfiguration.xml"
# we can work with search config at the tenancy or site collection level:
#$configScope = "SPSiteSubscription"
$configScope = "SPSite"

$searchConfigurationPortability = New-Object Microsoft.SharePoint.Client.Search.Portability.SearchConfigurationPortability($clientContext)
$owner = New-Object Microsoft.SharePoint.Client.Search.Administration.SearchObjectOwner($clientContext, $configScope)
	
[xml]$searchConfigXml = Get-Content $pathToSearchSchemaXmlFile
$searchConfigurationPortability.ImportSearchConfiguration($owner, $searchConfigXml.OuterXml)
$clientContext.ExecuteQuery()

Write-Host "Search configuration imported" -ForegroundColor Green