Is there a command in Linux which waits till it will be terminated?

I don't get why all the other replies want to use a loop here, sleep is able to wait long enough for any practical purpose and will use no clock cycles doing it.

For example it will try here to sleep for a dubious one hundred years and will likely do it enough to comply with the request anyway:

echo "Something"; sleep 36500d

Alternatively, this should block too until you type the Enter key:

echo "Something"; read foo

Try the -N option instead. From the plink documentation:

-N        don't start a shell/command (SSH-2 only)

This is the same behavior as the ssh option -N, as described in the man page:

-N      Do not execute a remote command.  This is useful for just for-
         warding ports (protocol version 2 only).

This should sleep forever without consuming any (noticeable) amount of CPU power.

echo "Something" && while true; do sleep 9999; done

I'm also not sure whether you can give a command like in the ForceCommand clause. You may need to put the command in a shell script.

#!/usr/bin/env bash
echo "Success! Close this window to log out." && while true; do sleep 9999; done

This script should of course be in a place and have permissions such that no ordinary user on the server can write to it.

Match User sampleUser
    ForceCommand /usr/bin/waitforever

Edit

I've found a command which seems more elegant:

echo "Something" && tail -f /dev/null

tail -f waits for a file or stream to return bytes. (Otherwise useful for watching logs in realtime.) /dev/null will never return bytes. And so the command will sleep forever.