Hi Fred, by setting the security level to production, a login page will be available to use. See Create a Secure App - Studio Pro 9 How-to's | Mendix Documentation for this.
If you need to customize the login logic you can use the Mendix Marketplace - SignIn microflow for Mx7 / Mx8.
Note. Mx9 not support but you can export from Mx8 and import to Mx9.
Hope to help.