You can use Hosting Controller Panel to create bulk mailboxes. But if you must have to create manually then use below Exchange PowerShell commands,
First run this command to apply password for newly added mailbox:
$Password=Read-Host “Enter Password” –AsSecureString
Execute this command to create mailboxes from CSV file, sample is added as an attachment
Import-CSV "c:\CreateMailboxes.csv" | ForEach {New-Mailbox -Alias $_.alias -Name $_.name -userPrincipalName $_.UPN -PrimarySMTPAddress $_.UPN -OrganizationalUnit "OU=TestDepartment,DC=ad10,DC=lab03,DC=com" -Password $Password}
Remember by adding mailbox manually will not apply any segregation and they will be listed to all users in GAL/AL