If you decide to build a Claim Your Gift page on your Webflow site, there's a bit more work, but with some great advantages;
How do I prevent Google from indexing my Claim Your Gift page and giving it to the world?
Use this META tag. It's easier than you think.
How do I prevent someone from posting a link to my Claim Your Gift page, and then everyone going directly there?
Realistically, you can't. But, Webflow has a nice page-level password feature that you can use.
Password-protect your page, and include both the link and the password in your email.
Of course, someone just has to post the password and the link together, and your security is defeated. So if you're really protective of your content, change that password regularly.
Make sure you change it in both places, simultaneously...