It’s possible only in development mode.
That is not great as the page will be in production mode 
User should do registration via shopware, but after registration you shouldn’t do authorization for this user. After customer register - you redirect him to your custom login page, where he sign in with 3rd service credentials, if customer in 3rd service exist - you should authorize this user via shopware.
User does not do any kind of registration. The user data is imported to a custom table and is part of shopwares database. So we are using table custom_users and checking if there is an entry that matches the 3 columns that were submitted with the „login“ request.
Everything is already working, I just need to move those 3 lines of code somewhere they will always be executed before page is loaded from cache. I’ll start digging in the shopware code today to find such a place and hope it exists.
Thank you for your time and energy spent trying to help me with this @odessite