DateTime to ISO8601UTC

$date.ToUniversalTime().ToString("yyyy-MM-ddTHH:mm:ss.fffZ") 
Details
Format a DateTime value in the ISO8601UTC format

Example
PS C:\> $Date = Get-Date
>> $date.ToUniversalTime().ToString("yyyy-MM-ddTHH:mm:ss.fffZ")

2019-03-28T16:06:00.050Z