To prompt for password when running a command over an ssh connect, allocate pseudo terminal using the -t flag
$ ssh -t remote.server "sudo reboot now"
[sudo] password for user:
Connection to remote.server closed.
Without using the -t flag, an error will appear:
$ ssh remote.server "sudo reboot now"
sudo: a terminal is required to read the password; either use the -S option to read from standard input or configure an askpass helper