I would send emails to NamUs and the investigating agency. For the investigating agency (since it will more than likely be read by the investigator(s) personally, I would feel more comfortable elaborating on why you think this is a match. Include some links, maybe outline with bullet points. I think some people have actually written mini presentations (like with Word or Google Docs) and then attached it to the email. Doe Network has a match form you fill out on their website.
I have submitted once before, in early January. Haven't heard anything back (aside from the automated "we got it" message), but I also did it via NamUs and Doe Network (not everyone has a profile on DN) and chose to wait before emailing the department directly. The latter is more intimidating. II have a match I really care about but I also have to fight the feelings of messing with something I shouldn't. Can you tell my fatal flaw is overthinking? Lol.