Here is a simple template you can use to write a third-party authorization letter.
This is to notify you that [Company] will be a part of all discussions taken after [date] as a third party authorized from our end. [Company] is authorized to work with all clients, receive and inspect information regarding business documents and applications on record.
Any communication sent to our Company shall also be marked to the third-party organization authorized in this letter and its assignees as a business request. [Company] will also be responsible for handling all necessary work, which has to be done in the course of business.
Request your valuable effort to formalize all details and proceed towards incorporating the change above.