pass form data to cart.php?a=view (or checkout) page

[cdeese8]

How do I pass a customer name field into the checkout form (cart.php?a=view) name field using WHMCS 7.4.* and standard six theme? Do you recommend I just a cookie instead of a GET / POST? https://stackoverflow.com/a/872522 -- this was the post I wanted to use at first: https://stackoverflow.com/a/43268025 and it didn't work, but I believe there is a security issue with adding PHP snippets directly onto custom pages or even wrapping them in the {PHP} tag. I'd prefer to do this as clean as possible and not going into WHMCS Admin Options to rig something.

I browsed the "Linking" docs but didn't have any luck with appending "&email=jeff@example.com" to a regular <a> link either. I think it only works for support tickets and does not work on checkout templates like cart.php?a=view ... https://docs.whmcs.com/Linking_to_WHMCS

> You can also define a name and email address:

Thanks for anything that might come to your mind.