Get ExchangeCustomAttribute via MS Graph

Get-MgUserExchangeAttributes.ps1

function Get-MgUserExchangeAttributes {
	param(
		[Parameter(Mandatory)]
		[Guid]$UserId
	)

	$attributes = (1..5).ForEach{"extension_79c86e5137c54776975dce9deb389ed4_extensionAttribute$_"}
	$result = Get-MgUser -UserId $UserId -Property $attributes -ErrorAction Stop
	return [PSCustomObject]@{
		UserId = $UserId
		ExtensionCustomAttribute1 = $result.AdditionalProperties[$attributes[0]]
		ExtensionCustomAttribute2 = $result.AdditionalProperties[$attributes[1]]
		ExtensionCustomAttribute3 = $result.AdditionalProperties[$attributes[2]]
		ExtensionCustomAttribute4 = $result.AdditionalProperties[$attributes[3]]
		ExtensionCustomAttribute5 = $result.AdditionalProperties[$attributes[4]]
	}
}