How do you address an application letter?
Address your letter to this person with a common business greeting, such as “Dear Mr./Ms.” and their last name. If you're unable to find their preferred gender pronouns (she/her, them/they) of the individual reviewing your application, you can use “Dear [first and last name]” or “Dear Hiring Manager.”
How do you address a reference letter?
If you are writing a personal letter of reference, include a salutation (Dear Dr. Smith, Dear Mr. Jones, etc.). If you are writing a general reference letter, say "To Whom it May Concern" or simply don't include a salutation.