React loading third party script and calling a function from that script after

Trying to load in a form from a third party service

They want us to load the script for their service then another script to populate the form with html/data

//load the marketo script if it doesn't exist already
const loadMarketoScript = (callback) => {
  const existingScript = document.getElementById('mktoForms');
  
  if (!existingScript) {
    const script = document.createElement('script');
    s.id = "mktoForms";
    s.type = "text/javascript";
    s.async = true;
    s.src = "//app-ab11.marketo.com/js/forms2/js/forms2.min.js";

    document.getElementsByTagName("head")[0].appendChild(script);
    script.onload = () => { 
      if (callback) callback();
    };
  }
  if (existingScript && callback) callback();
};

export default loadMarketoScript;

//page calling the function to load the script
  const [loaded, setLoaded] = useState(false);
  useEffect(() => {
    loadMarketoScript(() => {
      setLoaded(true);
    });
  });

  useEffect(() => {
    MktoForms2.loadForm("//748-KKO-677.mktoweb.com", "748-KKO-677", 1169);
  }, [loaded]);

However MktoForms2 shows as undefined. Unsure what to do here.