Wants to login to the backend canister deployed link using login button with principal id i get when i login in a frontend app

[Severin]

Principal_id another url

It will only be different when the origin is different, the definition of an origin can be found here Origin - MDN Web Docs Glossary: Definitions of Web-related terms | MDN In your subdomains example, both are different origins so they’ll result in different principals. If both origins are under your control, e.g. they’re part of the same dapp, you can use the alternative frontend origins config to have the same principal.