Hi Raja,
The login() function returns a Promise that resolves when the user with the specified email address and password is logged in.
Checkout the API documentation here - https://www.wix.com/code/reference/wix-users.html#login
You can create a custom login form in the editor, put it within a lingbox (same as you created for the custom signup). The login form is used to log a user in based on email and password, so you need add to your custom login form, an input field for login and one for password field, and add a link to connect between the login page and the signup page.
And here’s a link to an article for a basic example of working with the new registration APIs - Velo Tutorial: Creating a Custom Registration Form with Code | Help Center | Wix.com
https://www.wix.com/code/home/forum/wix-tips-and-updates/custom-member-registration
Hope this helps.
Thanks