How to rewrite main URL to sub-domain URL for white label solution?

Hello everyone,

I have a WP website that is available for people to register and join on their own through the main domain. However, I am also getting interest from some organizations to use a white label version of the product for their internal use but also willing to let users who join through their white label solution to access the main site if users want it.

In summary, they would like the URL to appear as if it is a unique link but the actual site opened would be the main site.

Based on this I have the following requirements

1. Create a subdomain for the client – client.site-name.com — easy to do
2. Use the same DB and tables for both subdomain and the main site. This removes the option of using muti site
3. When client users register and use the product, all URLs should show client.site-name.com/XYZ instead of site-name.com/XYZ but the content should load from the site-name.com/XYZ pages.

How can I accomplish this?

Things I have tried –

1. Domain masking — it creates an iframe which leads to the URL not changing in the browser when user navigates to different pages

Has someone done this before? Help appreciated?