In the "Bordericons" property of the login form, just leave marked as "True" the parameters "biSystemMenu" and "biMinimize".
Still in the login form declare the following Procedure:
...
private
{ Private declarations }
procedure WMSysCommand(var Message: TWMSysCommand); message WM_SYSCOMMAND;
...
implementation
{$R *.dfm}
procedure TFrmLogin.WMSysCommand(var Message: TWMSysCommand);
begin
if (message.cmdtype and $FFF0) = SC_MINIMIZE then
begin
EnableWindow(Application.handle, true);
Application.Minimize;
end else
inherited;
end;
To Procedure causes the entire application to be minimized by minimizing the form, even if it has been called with "Showmodal".
Note: Because it is a Login screen, check well for security reasons if there are no other ways for the user to close the screen without proper authentication.