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Every platform has a mechanism for its default platform. The LCL unit lazhelphtml contains a class THTMLBrowserHelpViewer to start a web browser for the LCL help system. You can use its method FindDefaultBrowser to find the default browser and the parameter to start. For example:

uses
  Classes, ..., LCLProc, LazHelpHTML;
 
...
 
implementation
 
procedure TMainForm.Button1Click(Sender: TObject);
var
  v: THTMLBrowserHelpViewer;
  BrowserPath, BrowserParams: string;
begin
  v:=THTMLBrowserHelpViewer.Create(nil);
  v.FindDefaultBrowser(BrowserPath,BrowserParams);
  debugln(['Path=',BrowserPath,' Params=',BrowserParams]);
  v.Free;
end;

This gives for example under Ubuntu (Linux):
 

Browser=/usr/bin/xdg-open Params=%s

Under Windows you can get:

Browser=C:\windows\system32\rundll32.exe Params=url.dll,FileProtocolHandler %s

Starting the browser

Once you know the command line and parameters you can use TProcessUTF8 to start the browser:

uses
  Classes, ..., LCLProc, LazHelpHTML, UTF8Process;
 
...
 
implementation
 
procedure TMainForm.Button1Click(Sender: TObject);
var
  v: THTMLBrowserHelpViewer;
  BrowserPath, BrowserParams: string;
  p: LongInt;
  URL: String;
  BrowserProcess: TProcessUTF8;
begin
  v:=THTMLBrowserHelpViewer.Create(nil);
  try
    v.FindDefaultBrowser(BrowserPath,BrowserParams);
    debugln(['Path=',BrowserPath,' Params=',BrowserParams]);
 
    URL:='http://www.lazarus.freepascal.org';
    p:=System.Pos('%s', BrowserParams);
    System.Delete(BrowserParams,p,2);
    System.Insert(URL,BrowserParams,p);
 
    // start browser
    BrowserProcess:=TProcessUTF8.Create(nil);
    try
      BrowserProcess.CommandLine:=BrowserPath+' '+BrowserParams;
      BrowserProcess.Execute;
    finally
      BrowserProcess.Free;
    end;
  finally
    v.Free;
  end;
end;