DM_SETHISTORY

The ID of the dialog item for which the history is changed.

Pointer to a string containing the history list ID, or NULL, if the history should be disabled.

FALSE - the dialog item specified in Param1 does not support history lists.
TRUE - the history has been assigned successfully.

Control	Description
DI_EDIT	edit line
DI_FIXEDIT	fixed-size input field

If the edit line is empty and the flag DIF_USELASTHISTORY for the DI_EDIT item is set, FAR will automatically subsitute the first value from the history into an empty edit line.

Implementation of this message can be seen in MultiArc plugin sources.

This example sets history and empties edit line:

...
  else if(Msg == MAM_ARCSWITCHES)
  {
    static char SwHistoryName[NM];
    FSF.sprintf(SwHistoryName,"ArcSwitches\\%s",pdd->ArcFormat);
    Info.SendDlgMessage(hDlg,DM_SETHISTORY,4,(LONG_PTR)SwHistoryName);
    Info.SendDlgMessage(hDlg,DM_SETTEXTPTR,4,(LONG_PTR)"");
  }
...

In order to make FAR substitute the first line from keys histiry the code should be rewritten in the following way:

...
  else if(Msg == MAM_ARCSWITCHES)
  {
    static char SwHistoryName[NM];
    FSF.sprintf(SwHistoryName,"ArcSwitches\\%s",pdd->ArcFormat);
    Info.SendDlgMessage(hDlg,DM_SETTEXTPTR,4,(LONG_PTR)"");
    Info.SendDlgMessage(hDlg,DM_SETHISTORY,4,(LONG_PTR)SwHistoryName);
  }
...

The edit line is being emptied and the item history is set after.