Author: Brody Kilpatrick | Created: April 17, 2019
$id = "MailboxYouWantToGrantAccessTo@mail.com"
$us = "UserThatNeedsAccess@mail.com"
Get-MailboxPermission -Identity $id -User $us | ft -AutoSize -Property Identity, User,AccessRights
Add-MailboxPermission -Identity $id -user $us -AccessRights FullAccess -AutoMapping $true
get-mailbox -Identity $id | Add-ADPermission -User $us -ExtendedRights "Send As"
Set-Mailbox -Identity $id -GrantSendOnBehalfTo $us