I tried it myself and the same thing was happening to me - got it working now though - looks like it’s case sensitive (possibly to emphasize that we’re using unique ASCII characters, where lowercase characters are differentiated from upper case). Try “GET” instead of “get”.
You probably have the right idea, just do it without the [ ] - those just indicate it’s a placeholder.
So if there’s a user (host) named franklee3 and you want their IP, then you would type the DNS’ IP address into the blue “To” box, then in the message, type
GET franklee3
and you should receive a message containing user franklee3’s IP address.