\typein[cmd]{msg}
Prints msg on the terminal and causes LaTeX to stop and wait for
you to type a line of input, ending with return. If the cmd
argument is missing, the typed input is processed as if it had been
included in the input file in place of the \typein command. If
the cmd argument is present, it must be a command name. This
command name is then defined or redefined to be the typed input.