Hello Kevin,
In Mendix we can use authentication widgets Sign-in Button to login, there are some setups in security and navigation.
Below I have attached all the links which may help you to create User login page, user Registration page And how to setup this setting.
I hope you resolve this issue by using this link.
Custom Login Page link:-
https://gettingstarted.mendixcloud.com/link/module/183/lecture/1426/3.7.3-Creating-a-Custom-Login-Page
Design Login Page link:-
https://gettingstarted.mendixcloud.com/link/module/183/lecture/1428/3.7.4-Designing-the-Login-Page
User Registration Link :-
https://gettingstarted.mendixcloud.com/link/module/183/lecture/1430/3.7.5-Allow-Anonymous-Users-to-Register
Mendix has a standard process for sign-in. What exactly do you want to achieve?