Try if the following suits you:
uses
Winapi.CommCtrl;
procedure TForm1.TreeView1Editing(Sender: TObject; Node: TTreeNode; var AllowEdit: Boolean);
var
EditHandle: THandle;
S: string;
begin
AllowEdit := True;
S := 'text to edit';
EditHandle := TreeView_GetEditControl((Sender as TTreeView).Handle);
SendMessage(EditHandle, WM_SETTEXT, 0, LParam(PChar(S)));
end;