VMS Help TCPIP Services, TELNET, TN3270 Keyboard Functions, DEF_KEY  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    VT100: Ctrl/K               VT200: Ctrl/K
    DEFINE_KEY Function: DEFINE_KEY

    Lets you interactively define or redefine a key. You get a prompt
    for the name of the key to define and for a function you want to
    assign to that key.