GUI Status Function Key doesn't t react

Hello,

i defined in the initialization event a own gui status.

PARAMETERS: p_name TYPE string.

INITIALIZATION.

SET PF-STATUS 'ZMC'.

I defined a function key(F5). If the user clicks on it the it should write 'hello'.  Although one can click on this buttion it doesn't anything. I tried to acces the function with AT PF05 and with

AT USER-COMMAND.

    CASE sy-ucomm.

      WHEN 'TEST' .WRITE 'OK'.

      WHEN OTHERS.

    ENDCASE.

Nothing appears. What did I wrong?