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

[Run]
Filename: "{app}\temp\installation_files\license.exe"; BeforeInstall: SetEnvPath;

[Code]
#ifdef UNICODE
  #define AW "W"
#else
  #define AW "A"
#endif
function SetEnvironmentVariable(lpName: string; lpValue: string): BOOL;
  external 'SetEnvironmentVariable{#AW}@kernel32.dll stdcall';

procedure SetEnvPath;
begin
  if not SetEnvironmentVariable('PATH', ExpandConstant('{app}')) then
    MsgBox(SysErrorMessage(DLLGetLastError), mbError, MB_OK);
end;