Philip,
in R77.30 go to the Multi-Domain Security Management Properties, in that window you have Custom commands on the left, select that one and add a new command lets say use putty, enter the name SSH2 for it, add the "path\putty.exe" (when you have spaces in the path add the quotes) in the executable field and add "/L admin <IP>" (without quotes) in the params field.
Now go to the Domain Contents view or the Network Objects view in the general tab and right click on any gateway object :
This is what you see, except you will only see the command names you entered.
There is no option in R80.10 on domain level to create this. Neither can you select the SSH application, for the browser I presume the selection is done through the OS preference.
Regards, Maarten