How to create samaccountname with maximum 12 character? - powershell

Question

I need to create users with max 12 characters.
The last name can contain the first 11 characters (max) + the first letter of the first name
I'm not sure what to add next to $LastName
Can you help me with this?
$OU = "OU=Users,DC=domain,DC=com"
$PW = "Welcome$(Get-Random -minimum 0001 -maximum 9999)!"
$FirstName = Read-Host "Please enter the Firstname for the new user"
$LastName = Read-Host "Please enter the Lastname for the new user"
$AccountName = $LastName + $FirstName[0] # Last name (11 caracter max) + first character of first name
# test if a user with that accountname already exists and if so, append a sequence number
$count = 1
while (Get-ADUser -Filter "SamAccountName -eq '$AccountName'") {
$AccountName = '{0}{1}{2}' -f $LastName, $FirstName[0], $count++
}
$UPN = "$($FirstName).$($LastName)#domain.com" #if the upn exists, add a digit to the last name
# test if a user with that UserPrincipalName already exists and if so, append a sequence number
$count = 1
while (Get-ADUser -Filter "UserPrincipalName -eq '$UPN'") {
$UPN = '{0}.{1}{2}#domain.com' -f $FirstName, $LastName, $count++
}
# create a Hashtable for splatting parameters
$userParams = #{
Name = "$($LastName), $($FirstName)"
DisplayName = "$($LastName), $($FirstName)"
FirstName = $FirstName
LastName = $LastName
SamAccountName = $AccountName
OnPremisesOrganizationalUnit = $OU
UserPrincipalName = $UPN
Password = $PW | ConvertTo-SecureString -AsPlainText -Force
ResetPasswordOnNextLogon = $true
Archive = $true
}
# Create User On-Premise and move to the managers OU if possible
try {
New-RemoteMailbox #userParams -ErrorAction Stop
}
catch {
Write-Error $_.Exception.Message
}