Hi @efdp,
Welcome to the Auth0 Community!
I understand you would like to create users and add them to applications on creation.
First, let me clarify that the Connection name is a required field when creating users. Therefore, when creating users, you can determine which applications they belong by checking the Database Connections enabled for the applications.
I hope this answers your question.
Please reach out if you have any additional questions. I would be glad to help!
Thank you.