procedure ReceiveMessage (var Msg: TMessage);
message WM_COPYDATA;
...
procedure TFormReceive.ReceiveMessage;
var
 pcd: PCopyDataStruct;
begin
 pcd := PCopyDataStruct(Msg.LParam);
 Caption := PChar(pcd.lpData);
end;

procedure TFormXXX.Button1Click(Sender: TObject);
var
 cd: TCopyDataStruct;
begin
 cd.cbData := Length(Edit1.Text) + 1;
 cd.lpData := PChar(Edit1.Text);
 SendMessage(FindWindow('TFormReceive', nil), WM_COPYDATA, 0, LParam(@cd));
end;