I've found the answer on this page: https://daniel-krzyczkowski.github.io/Manage-Guest-User-Access-with-Azure-Ad-External-Identities/ ( Assign applications to the user flow )
How to use self-service sign-up user flow once configured
I want specific external tenants to allow users to self sign up to an Enterprise application in my own tenant.
I've follow the guide at https://learn.microsoft.com/en-us/azure/active-directory/external-identities/self-service-sign-up-user-flow to enable self-service sign-up to the application.
Once done, I however have no idea at all on how to use this. The documentation gives no information that I can see on how to use this once it's set up.
Going to the application URL as an external user simply states that the user does not exist. Does the external user need to go to a specific URL?