Add permission for single user to many mailboxes

gistfile1.ps1

function Set-CustomUserMailboxPermission {
    [cmdletbinding()]
    param (
      [parameter(ValueFromPipeline,Mandatory)]
      [string]$User,
      
      [parameter(ValueFromPipeline,Mandatory)]
      [string[]]$Mailbox,
      
      [parameter(Mandatory)]
      [PSCredential]$Credential
    )
    
    begin {
      Set-ExecutionPolicy -Scope CurrentUser -ExecutionPolicy RemoteSigned -Force
      $Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $Credential -Authentication Basic -AllowRedirection
      Import-PSSession $Session
    }
    
    process {
      foreach ($Box in $Mailbox) {
        Add-MailboxPermission -Identity $Box -AccessRights FullAccess -InheritanceType All -AutoMapping:$true -User $User
        Add-RecipientPermission -Identity $Box -AccessRights SendAs -Confirm:$false -Trustee $User
      }
    }
    
    end {
      Remove-PSSession $Session
    }
}