Additional field - link

Hi,

The "attribute" setting is for when the custom field is displayed in a form. Not when the system displays the value entered previously in the custom field.
So you should see the attribute added to the input field on the product edit page, not on the product details page on the frontend.

And yes, the default value is only the default value set in the custom field when displayed in the form.
It won't be set in all the products already existing.
That's possible to add it with a mass action.

However, the rel won't work. If you want to add the same link to all the products, rather than a custom product field, it would be better to directly add the HTML of the link in the show_default view file, via the menu Display>Views. That way, you'll have no problem also having the attributes you want in the link.