MyUser.cls

public class MyUser {
    public String username;
    public String password;
    
    public void validateCredentials() {
        // calling an static method without creating a new instance of MySecurityClass
        Boolean authenticated = MySecurityClass.validateCredentials(username, password);
        
        System.debug('User username: ' + username);
        System.debug('User Authentication: ' + authenticated);
    }
}