What is the script for send on behalf permission?

check if this helps,

$mailbox = Get-Mailbox -Identity MYSHAREDMAILBOX

$identity = $mailbox.UserPrincipalName

$users = Import-Csv -Path "C:\path\members.csv" -Delimiter ";"

foreach($user in $users){

    try{

        $setPermissions = Set-Mailbox -Identity $identity -GrantSendOnBehalf $user

        Write-Host "Successfully added permissions for $user" -ForegroundColor Green

    }catch{

        Write-Host "Failed to add permissions for $user" -ForegroundColor Red

    }

}

~Bart