I would send it to both of those names via PM. If you go into your messages and open the threatening message, then click "reply" and take out the name of the person who sent it to you, you can enter "forum moderator" and then click "add recipient" to enter JacobJ, then send it. And I can't even imagine why anyone would do that.
Since I wrote my original post, I have been doing reading on various topics on the Forum. One included a post from the person who threatened me and the moderator left a note: previously blocked poster has used numerous identities and IPs. Here's another.
So the moderator already knows, blocked them so I am not going to worry.