Not sure what's going on other than suspecting the quality of connection over the internet. But I can at least tell the "password" at the end of the command is currently not used by FMG (AFAIK with 6.4.7) so we use any dummy one like "preshared-key" or whatever. You just need to have something there.
Just make sure fgfm(CLI)/FMG-access(GUI) is enabled on the wan interface(s) and test quality of connection with pinging from both ends.
I tried it with a dummy password. You are right. I have a new device for authorization but the follow is the same. The authorization hangs. If I do this over the GUI, there are no problems. I am not sure that I understod you about "Just make sure fgfm(CLI)"