You set the conditions for a mouse port report by passing an iostdreq to
the device with ind_setmtrig set in io_command, the address of a
GamePortTrigger structure set in io_Data and the length of the structure
set in io_Length.

   struct GamePortTrigger InputTR;

   InputIO->io_Data = (APTR)InputTR;     /* set trigger conditions */
   InputIO->io_Command = IND_SETMTRIG;   /* from InputTR */
   InputIO->io_Length = sizeof(struct GamePortTrigger);
   DoIO(InputIO);

The information needed for mouse port report setting is contained in a
GamePortTrigger data structure which is defined in the include file
devices/gameport.h.

   struct GamePortTrigger
   {
       UWORD    gpt_Keys;      /* key transition triggers */
       UWORD    gpt_Timeout;   /* time trigger (vertical blank units) */
       UWORD    gpt_XDelta;    /* X distance trigger */
       UWORD    gpt_YDelta;    /* Y distance trigger */
   };

See the gameport device chapter of this manual for a full description
of setting mouse port trigger conditions.