How to force users to subscribe without creating an account?

I've created a web app which does not require a user to create an account, you just fill in a form and get the result.

I thought that would be nice to add "Enter your email" input text along with the CTA of my landing page (a "Try now for free" button), in order to force the users to subscribe my mailing list in exchange of using my web app.

My concern is that with that approach every time a user refreshes the page would have to enter their email again to use the app.

I came up with the following fixes:

  1. Store their email in a cookie so the input text gets automatically filled in.
  2. Asking for their email at the end of the app form.
  3. Sending a unique URL with the results page to their email (this would increase the interaction cost, but all emails would be verified).
  4. Don't make it mandatory, just add some CTA in the landing or result pages.

What option do you think is best? Do you have a better alternative?