The Object::useCustomClass() needs to be in your _config.php. This needs to be in CustomLoginForm.php:

class MyLoginForm extends MemberLoginForm { 
public function __construct($controller, $name, $fields = null, $actions = null, $checkCurrentUser = true) { 
parent::_contstruct($controller, $name, $fields, $actions, $checkCurrentUser); 
$this->Fields()->fieldByName('Email')->setValue('Some default value'); 
} 
}

You mean the config should look like this?

Object::useCustomClass('MemberLoginForm', 'CustomLoginForm');
Object::add_extension('Group', 'GroupDecorator');
Object::useCustomClass('MemberLoginForm', 'MyLoginForm');

Placed you function inside CustomLoginForm.php

but website gives error :(

What to do?

What does "gives error" mean?

The normal error from silverstripe when you have an error in your code.

You know:

WEBSITE ERROR

on a grey background in a blue-ish square.

Get your site into dev mode so we can see the error.

Director::set_environment_type('dev');

You shouldn't be running in live mode while you're still developing.

[User Error] Uncaught ReflectionException: Class MyLoginForm does not exist
GET /bedrijfsprofiel

Line 89 in /public/sites/www.url.nl/sapphire/core/Object.php
Source

80 * @param string $class the class name
81 * @param mixed $arguments,... arguments to pass to the constructor
82 * @return Object
83 */
84 public static function create() {
85 $args = func_get_args();
86 $class = self::getCustomClass(array_shift($args));
87
88 if(version_compare(PHP_VERSION, '5.1.3', '>=')) {
89 $reflector = new ReflectionClass($class);
90 return $reflector->newInstanceArgs($args);
91 } else {
92 // we're using a PHP install that doesn't support ReflectionClass->newInstanceArgs()
93
94 $args = $args + array_fill(0, 9, null);
95 return new $class($args[0], $args[1], $args[2], $args[3], $args[4], $args[5], $args[6], $args[7], $args[8]);

Did you create MyLoginForm.php?

At first i didn't create the MyLoginForm php, because the function was added on rule 1 of CustomLoginForm.php

Now i added the function to MyLoginForm.php - and deleted the function from customloginform.php

After dev/build

When I enter the page:

[User Error] Uncaught Exception: Object->__call(): the method '_contstruct' does not exist on ''
GET /bedrijfsprofiel

Line 576 in /public/sites/www.url.nl/sapphire/core/Object.php