Issue setting mailboxpermission based on user input

I'm trying to create a powershell script for our first line support to help them with the account creation process in Office 365.
One step of the script is to get a list of shared mailboxes and ask the user to insert which shared mailboxes the new account should have access to.

I've copied a previous script for a similar task when adding a new account to certain groups, but I can't get this one to work.

I'm very new to powershell scripting so it might be a very small issue or maybe I should attack it from a whole different angle.

Here's the script:

#Prompt user for shared mailboxes
Get-Mailbox -Filter '(RecipientTypeDetails -eq "SharedMailbox")' | Select Alias | Format-Table
$sharedmailboxes = Read-Host "Copy the shared mailboxes the account should access. Separate multiple mailboxes with comma (,)"
$sharedmailboxdata = $sharedmailboxes.Split(",").Trim(" ")

#Adding new account to shared mailboxes
Write-Host -BackgroundColor Black -ForegroundColor Yellow "Adding account to shared mailboxes"
ForEach ($line in $sharedmailboxdata) {
$sharedmailboxobject = Get-Mailbox -RecipientTypeDetails Shared | Select-Object $line
Add-MailboxPermission -Identity $sharedmailboxobject -User $UserPrincipalName -AccessRights FullAccess -InheritanceType all
}

With this script I get the following error:

Cannot process argument transformation on parameter 'Identity'. Cannot convert the "System.Collections.ArrayList" value of type "System.Collections.ArrayList" to type "Microsoft.Exchange.Configuration.Tasks.MailboxIdParameter".
    + CategoryInfo          : InvalidData: (:) [Add-MailboxPermission], ParameterBindin...mationException
    + FullyQualifiedErrorId : ParameterArgumentTransformationError,Add-MailboxPermission
    + PSComputerName        : ps.outlook.com