...
uses
  ...
  FileAssoc;//<-- add fileassociation unit here

type

...
    { private declarations }
    assoc: TFileAssociation;
...

procedure TfrmMain.FormCreate(Sender: TObject);
begin
  assoc := TFileAssociation.Create(Self);//<-- create like a regular component
end;

procedure TfrmMain.Button1Click(Sender: TObject);
begin
  assoc.ApplicationName := 'Lazarus IDE';
  assoc.ApplicationDescription := 'RAD for Free Pascal';

  // you can change Extension and Action part for each extension you have

  assoc.Extension := '.lpr';
  assoc.ExtensionName := 'Lazarus Project';
  assoc.ExtensionIcon := '"C:\lazarus\images\lprfile.ico"';

  // full path required, you can use ParamStr(0) to get the path with the .exe name included. The path must be inside quotes if it has whitespace.
  assoc.Action := '"C:\lazarus\lazarus.exe" "%1"'; 
  assoc.ActionName := 'Open';
  assoc.ActionIcon := '"C:\lazarus\images\mainicon.ico"';

  // notice that using RegisterForAllUsers as True requires Administrator Privileges
  // if you want to run without privileges set it to false, but it will register for current user only
  assoc.RegisterForAllUsers:=False;
  if assoc.Execute then
  begin
    ShowMessage('OK');
    assoc.ClearIconCache; //<<-- rebuild icons
  end;
end;

end.

procedure TForm1.FormCreate(Sender: TObject);
var
  s: String;
begin
// if there are parameters
  if ParamCount > 0 then
  begin
// load the first parameter
    s := ParamStr(1);
 
// if is a .txt file
    if ExtractFileExt(s) = '.txt' then
    begin
// load the .txt file into a memo
      Memo1.Lines.LoadFromFile(s);
    end;
  end;
end;