To allow Exchange Recipient Administrators the ability to manage mailbox permissions the following needs to be ran

I just tested this in my test environment. You can run below command which gives "Administer Information Store" (ms-Exch-Store-Admin) rights to your helpdesk staffs that allow them to give full access rights to all users of your Exchange organization.

Add-ADPermission -Identity "CN=CompanyOrg,CN=Microsoft Exchange,CN=Services,CN=Configuration,DC=company,DC=com" -User "company\HelpDeskStaffs" -ExtendedRights ms-Exch-Store-Admin -InheritanceType All