How to modify deafult fields specifically on front-end User Registration Form

Okay, follow up question:

I have followed the examples you shared, and ended up with the resulting code:

//adapted from: https://support.metabox.io/topic/mb_user_profile_register-autocompletenew-password/
//relevant docs: https://docs.metabox.io/extensions/mb-user-profile/#form-fields-filters
function add_more_registration_fields( $fields ) {
        $fields = [
			'first_name' =>[
                'id'   => 'first_name', // THIS
                'name' => 'First Name',
                'type' => 'text',
				'required' => true,
				'columns' => 4,
            ],
   			'last_name' =>[
                'id'   => 'last_name', // THIS
                'name' => 'Last Name',
                'type' => 'text',
				'required' => true,
				'columns' => 4,
            ],
            'username'  => [
                'name'     => __( 'Username', 'mb-user-profile' ),
                'id'       => 'user_login',
                'type'     => 'text',
                'required' => true,
				'columns' => 4,
            ],
            'email'     => [
                'name'     => __( 'Email', 'mb-user-profile' ),
                'id'       => 'user_email',
                'type'     => 'email',
                'required' => true,
				'columns' => 6,
				'desc' => 'Used for notifications and alerts (if opted-in).',
            ],
			'phonenumber'  => [
            'name'     => __( 'Phone Number', 'mb-user-profile' ),
            'id'       => 'phonenumber',
            'type'     => 'tel',
			'required' => true,
			'desc' => 'Used for notifications and alerts (if opted-in).',
			'columns' => 6,
        	],
			'password'  => [
                'name'     => __( 'Password', 'mb-user-profile' ),
                'id'       => 'user_pass',
                'type'     => 'password',
                'required' => true,
				'columns' => 6,
                'desc'     => '<span id="password-strength" class="rwmb-password-strength"></span>',
                'attributes' => [
                    'autocomplete' => 'new-password'
                ]
            ],
            'password2' => [
                'name'     => __( 'Confirm Password', 'mb-user-profile' ),
                'id'       => 'user_pass2',
                'type'     => 'password',
                'required' => true,
				'columns' => 6,
            ],
            // ... add more fields here
        ];
        return $fields;
    }
add_filter( 'rwmb_profile_register_fields',  'add_more_registration_fields' );

Everything works except the first 2 fields. The first and last name do not get copied into the user meta. I am following the instructions listed here: https://docs.metabox.io/extensions/mb-user-profile/#edit-default-fields. The IDs match. Is there something else I'm doing wrong?

Thank you, that will work for now. It would be helpful to have this functionality in the future, as it would allow the actual set of default fields to be ordered alongside the others in the registration form.