[Setup]
AppName=My Program
AppVersion=1.5
DefaultDirName={pf}\My Program

[Code]
function GetServerID(Param: string): string;
begin
  Result := 'your server :)';
end;

procedure InitializeWizard;
var
  S: string;
begin
  S := ExpandConstant('Hello, I''m {code:GetServerID}');
  MsgBox(S, mbInformation, MB_OK);
end;