Place that code inside onClick function of your login button
import wixUsers from 'wix-users';
let email =$w('id_of_email_inputfield').value // email address of user to log in
let password =$w('id_of_password_inputfield').value // password of user to log in
wixUsers.login(email, password)
.then( () => {
console.log("User is logged in");
} )
.catch( (err) => {
console.log(err);
} );