Drop off library permission

My client has a drop off library on a site collection (HR) which is separate to their Intranet. Users can send documents to this drop off library, however they should not be able to view this library or the site itself.

Currently my client receives a ‘Let us know why you need access to this site’ message when sending a document (the document is sent successfully to the drop off library) through ‘Sent to’. What’s the lowest permission I can give the users to this library/site to stop this message appearing? Bear in mind that the users should not be able to see anything on this site or drop off library.

Thanks for any help!