He's looking for the command line args, not the URL.
I'm not sure how the - and _ are handled during the part where it goes from URL -> Command line, but I think just putting join-74.52.20.186_28100 where you'd normally put something like -editor or -trace would work. I'll test this.
Edit: Nope, entire URL in the command line.
Example:
"C:\Blockland\Blockland.exe" blockland://join-74.52.20.186_28100/