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Title: Execute a Custom Action on the Form's Help button Click (biHelp in Delphi)
If the form's BorderStyle property is bsDialog and biHelp is included in BorderIcons, a question mark appears in the form's title bar and when clicked, the cursor changes to crHelp.
If you want to execute some custom code when the user clicks the Help button, you need to handle two Windows messages: WM_NCLBUTTONUP and WM_NCLBUTTONDOWN. Those messages are posted to the window when the user presses and releases the left mouse button while the cursor is within the non-client area of a window.
Here's a simple implementation of the custom biHelp-click action:
type
THelpForm = class(TForm)
private
procedure WMNCLBUTTONDOWN(var Msg: TWMNCLButtonDown) ; message WM_NCLBUTTONDOWN;
procedure WMNCLBUTTONUP(var Msg: TWMNCLButtonUp) ; message WM_NCLBUTTONUP;
end;

var
HelpForm: THelpForm;

implementation
{$R *.dfm}

procedure THelpForm.WMNCLBUTTONDOWN(var Msg: TWMNCLButtonDown) ;
begin
if Msg.HitTest = HTHELP then
Msg.Result := 0 // "eat" the message
else
inherited;
end;

procedure THelpForm.WMNCLBUTTONUP(var Msg: TWMNCLButtonUp) ;
begin
if Msg.HitTest = HTHELP then
begin
Msg.Result := 0;
ShowMessage('Need help?') ;
end
else
inherited;
end;