Addressing letters of interests to "the HR Department" will not make your letter stand out or seem professional. If a position has recently opened up, write the letter to those in charge of hiring. If one hasn't, write the letter to your supervisor. Include their abbreviated professional title (e.g. "Dr.") if relevant.
How do you write a letter of interest for a job?
How do you write an email for a job interest?
Dear [NAME], My name is [YOUR NAME], and I noticed your work experience with [NAME OF COMPANY]. I follow you on [SOCIAL MEDIA SITE] and noticed and we're both are interested in [TOPIC]. I find your posts insightful and helpful.
How do you write a good statement of interest?
Ideally, the statement will address (1) your interest in the field/industry/employer, (2) how the shadow will help your career exploration process, and (3) provide a general idea of your travel plan for the job shadow.