Convert ImmutableID to Hex for AD

To get the immutableID value from a user (should be able to do similar with Get-MSOLUser if preferred):

$id = (Get-AzureADUser -ObjectId User@domain.com).immutableid

To convert that ID to hex for entry

$hex=([system.convert]::FromBase64String("$id") | ForEach-Object ToString X2) -join ' '

To view the value in $hex:

$hex

The immutable id will be a value something like: fhG+Kox7LkaYwSIf6s6UFA==

The hex for that one is: 7E 11 BE 2A 8C 7B 2E 46 98 C1 22 1F EA CE 94 14

The hex value can be entered into the ms-DS-ConsistencyGUID attribute of the user object.

And converting from objectGUID to ImmutableID

$immutableID = [system.convert]::ToBase64String(([GUID]($u.ObjectGUID)).tobytearray())