Include third-party library
You can add your library in your file theme.libraries.yml (or module.libraries.yml) like this:

# Parsley use for contact_form.
parsley:
  js:
    dist/js/vendors/parsley.min.js: { minified: true }
  dependencies:
    - core/jquery


After, you can include your library like any other libraries with a hook or a template.