// handle button click to call a modal form
procedure TfmMain.btnEnterDMCClick(Sender: TObject);
begin
 EnableSearchButtons(false);
 fmEnterDMC            := TfmEnterDMC.Create(NIL); // create the form
 fmEnterDMC.OnClose    := OnDmcClose; // the OnClose handler for later freeing the closed modal form
 fmEnterDMC.Enter; // call a member procedure that calls ShowModal
end;

// free the closed modal form when it returns
procedure TfmMain.OnDmcClose(Sender: TObject);
begin
 ShowMessage('OnDmcClose'); 
 fmEnterDMC.Free;
 EnableSearchButtons(true);
end;
-----------------------------------
// in the called form call ShowModal
procedure TfmEnterDMC.Enter;
begin
 ShowModal;
end;

Thert is a button in the modal form, when clicked calls "Close".
procedure TfmEnterDMC.btnSearchClick(Sender: TObject);
begin
 Close;
end;