Hello Filippo Iacobellis,
Thank you for posting your question in the Microsoft Q&A Community.
There are a few steps to follow to achieve this.
Just like you have it in the other Azure tenant, you need to create an application (under App registration) in the Azure B2C tenant. In the authentication section, add your redirect URL. Follow the link below to get more information about this.
Once that is done, create your user flow and select the application you created above.
To now integrate this with your code, depending on the language you have built your application you need to add the application ID of the application created earlier, the B2C tenant ID, and the user flow name. Follow this link to take a look at the sample codes on how to integrate user flow with your application.
https://learn.microsoft.com/en-us/azure/active-directory-b2c/integrate-with-app-code-samples
Let me know if further clarification is needed.
Babafemi