Use the DEF KEY function to define or redefine a key
    interactively. Your new definition exists until you log out from
    the remote host or disconnect from it.

    When you invoke the DEF KEY function, TN3270 displays a prompt
    in the status line at the bottom of your screen. What you enter
    during the DEF KEY dialog is subject to translation from the
    National Character Set to the DMCS.

    You cannot redefine a key that exists on your terminal if it
    lacks a DMCS equivalent.