Create a CSV with following fields and save it in a temp folder

UserMailbox and User

copy the below and save as .ps1 file in the same temp folder e.g C:\temp

$csv = Import-csv -path “C:\temp\fullaccess.csv”
foreach($User in $csv)
{
Add-MailboxPermission -Identity $user.UserMailbox -User $user.User -AccessRights FullAccess -InheritanceType All
}

 

Run the script to apply permissions