Customising Login Form Field SS4

Questions
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1

Silverstripe 4

I would like to customise form field with custom class.

On prev version, I could extend MemberLoginForm with construct overriding the form field and action.

Object::useCustomClass(‘MemberLoginForm’, ‘CustomLoginForm’);

How do I customise login form in SS4 ?

I tried to create custom authenticator, and add injector.

However I receive the following error:
Error: Call to a member function supportedServices() on string

Line that break is:

$authenticator->;supportedServices()

Which I am not sure why I got string there.

This is my injector

---
Name: mechwestsecurity
After:
  - '#coresecurity'
---
SilverStripe\Core\Injector\Injector:
  SilverStripe\Security\Security:
    properties:
      Authenticators:
        myauthenticator: %$MyAuthenticator