My first internship was in an HR department. I used to scan resumes into the database all day. And yes, we didn't like it when people called to get the name of the hiring manager.
That being said, when a resume was forwarded to us by a hiring manager, that automatically gave the resume more priority. We didn't want to be accused of losing a resume, or filtering out a resume.
Please understand the nuance I'm making. The resume didn't just need to "addressed to the hiring manager", it needed to be forwarded from the hiring manager, or if not forwarded by him/her, it needed to have something written in the email or in the cover letter that implied that the candidate had already spoken to the hiring manager.
So any resume sent to us that way, we'd automatically tick a checkbox in the database after we scanned it in, and for paper copies, we made sure those resumes were placed on the very top of the stack when sending the stack in. The checkbox in the database had the same effect, it would help place the resume on top of the list in the actual database.
For that reason, I always include the name of the hiring manager with my resume and I always use the email address (or the maildrop) of the hiring manager. That being said, if the hiring manager tells me to upload the resume directly into their portal, I will obviously follow their instructions. It's just that I'll be sure to include a note that says that so-and-so told me to upload the resume to the portal.
If the name of the hiring manager is not available, the other thing that works is to use another employee's name (not in HR). At my old job, that had the same effect. Again, we didn't want to be accused of losing resumes, or filtering out resumes. So if it looked like a resume came in from an employee referral, we'd make sure it got top priority as well (whether or not the employee was the hiring manager or not).
And finally, what happens if I can't easily get the name or email address of anyone outside of HR? I generally don't sweat it. My own resume is generally good enough to get interviews.
In fact, if your own resume is good enough to get you interviews in general, you should feel free to completely ignore my advice. As long as you're able to get to the next stage, that's all that really matters.