Provide email to access website

Site URL?

https://www.fresaskonkrema.com

Do you mean to restrict usage to only members?

you can custom code it, like get the value on a session variable and your theme loop check if variable is there then display the content else overlay.

What the customer exactly wants is a landing page or a lightbox over the main page and dont let the user access the main page until his email is provided, and the email is added to a mailing list.
Thank you for your interest!

I think you may install a plugin like the ones here.

It seems a little complicated let me take a look, thank you!

But could be better find a way to enforce provide the user email to access the main page

Did you try the General Settings on dashboard (Membership check box)?