Sending metafield data with form

[FilmBro]

Hi community,

This is what I am trying to do: 

I have free and paid products on my website.
For free products, people need to go through checkout, which is a bad experience.

So I had this idea:
I want to change the 'add to cart' button to a download button.

And when people click the download button, a pop-up opens with a field to leave their name & email, and the download will be sent straight to their inbox.

This is where I am at now:

The 'add to cart' button for free products is now a 'download free' button.
When someone clicks the button, the pop-up shows to fill in your name & email.

+ I have a hidden form field for the download link that I want to connect to a metafield with the free download link, so the email system knows which free download link to send to their inbox.

This is where I struggle:

I am unable to use the hidden form field for the download link with my metafield.

(I added a free download link metafield in the product backend).
It seems like I am unable to use the metafield code to send the specific download link with the form.

Is there anyone that might be able to help?

Thank you in advance.