This core plugin is exactly for your described use case. Activate it and you get a check box in your contact form and with the link to the article that you can add here.
Hope this helps.

@LadySolveig nice secret gem you shared there thank you.

However my sites are multilingual and it won't fit for it:
I tried as I have a consent policy article in french and english(and the articles are associated), however when selecting the french article from the plugin, it won't auto redirect to the english associated language of the article.

@joomlafun Thank you, I have just tested it, you are right. But you can use the menu selection instead of articles as a workaround.
Simply create a menu item for each language and set them to hidden (Tab -> Link Type | Display in Menu switch to No) so that they don't appear anywhere. This way the redirection to the corresponding language will work.

Not really a hidden secret, but with multilanguage you should take care to link as much as possible to the menu. This way the redirects are simply handled more robustly. :)

Somehow I did not see that I could link to a menu item and it does indeed changes everything regarding language redirection.

Thanks a lot again LadySolveig for the explanations, I am sure that your kind answers will serve other who are looking to implement properly a solution for the consent form.