redirection – How do I force /web in the URL?

I have installed Drupal 8 as multi-site using Composer (as described by kalose). Basically, the folders created as subdomain-folders have been replaced with a symbolic link to the respective sub-domain folders in /sites.
I have 2 websites on 2 sub-domains, sub1.example.be/web and sub2.example.be/web.

In the public_html directory, I placed an index.php file with the following code.

header ("Location: https://sub1.example.be/web");

It works fine.

Where can I put a similar index.php file to redirect users who use the old link (sub2.example.be) to sub2.example.be/web?
Is there any other alternative to achieve the same?