Add Custom field in woocommerce checkout page (Woocommerce मैं नया select box / Checkbox जोड़ना )

is_woocommerce() : check is woocommerce 

is_cart() : check is cart 

is_checkout() : check is checkout page 

1.  Add News Letter Check Box in woo commerce checkout page 

<?php

    class WooSubscribeCheckbox {

    // add hooks into WooCommerce

    public static function run() {

        add_filter('woocommerce_checkout_fields',

            array(__CLASS__, 'filterWooCheckoutFields'));

        add_action('woocommerce_checkout_update_order_meta',

            array(__CLASS__, 'actionWooCheckoutUpdateOrderMeta'));

        add_filter('woocommerce_email_order_meta_keys',

            array(__CLASS__, 'filterWooEmailOrderMetaKeys'));

    }

    /**

    * add custom fields to WooCommerce checkout

    * @param array fields

    * @return array

    */

    public static function filterWooCheckoutFields($fields) {

        global $woocommerce;

        // add field at end of billing fields section

        $fields['billing']['our_mailing_subscribe'] = array(

            'type' => 'checkbox',

            'label' => 'Subscribe to mailing list?',

            'placeholder' => 'Subscribe to mailing list',

            'required' => false,

            'class' => array(),

            'label_class' => array(),

        );

        return $fields;

    }

    /**

    * save custom order fields

    * @param int $order_id

    */

    public static function actionWooCheckoutUpdateOrderMeta($order_id) {

        $subscribe = isset($_POST['our_mailing_subscribe']) ? 'yes' : 'no';

        update_post_meta($order_id, 'Subscribe to mailing list', $subscribe);

    }

    /**

    * add our custom fields to WooCommerce order emails

    * @param array $keys

    * @return array

    */

    public static function filterWooEmailOrderMetaKeys($keys) {

        $keys[] = 'Subscribe to mailing list';

        return $keys;

    }

}

WooSubscribeCheckbox::run();

Edit: how to auto-tick the checkbox:

add_filter('woocommerce_checkout_get_value', function($value, $input) {

    if ($input == 'our_mailing_subscribe') {

        $value = 'yes';

    }

    return $value;

}, 10, 2);

For More information check out this link 

https://snippets.webaware.com.au/snippets/subscribe-to-mailing-list-in-woocommerce-checkout/










2. Add select (Drop Down) in woo commerce Checkout Page 




/**
 * Add the field to the checkout
 **/
add_action('woocommerce_after_order_notes', 'my_custom_checkout_field');

function my_custom_checkout_field( $checkout ) {

    echo '<div id="my_custom_checkout_field"><h3>'.__('My Field').'</h3>';

    woocommerce_form_field( 'my_field_name', array(
        'type'          => 'select',
        'class'         => array('my-field-class form-row-wide'),
 'required'   => true,
        'label'         => __('Fill in this field'),
        'placeholder'       => __('Enter something'),
 'options'     => array(
        'choice1' => __('choice1', 'woocommerce' ),
        'choice2' => __('choice2', 'woocommerce' )
        )
        ), $checkout->get_value( 'my_field_name' ));

    echo '</div>';

}

/**
 * Process the checkout
 **/
add_action('woocommerce_checkout_process', 'my_custom_checkout_field_process');

function my_custom_checkout_field_process() {
    global $woocommerce;

    // Check if set, if its not set add an error.
    if (!$_POST['my_field_name'])
         $woocommerce->add_error( __('Please enter something into this new shiny field.') );
}

/**
 * Update the order meta with field value
 **/
add_action('woocommerce_checkout_update_order_meta', 'my_custom_checkout_field_update_order_meta');

function my_custom_checkout_field_update_order_meta( $order_id ) {
    if ($_POST['my_field_name']) update_post_meta( $order_id, 'My Field', esc_attr($_POST['my_field_name']));
}




 For more Details 

    https://wordpress.org/support/topic/checkout-custom-fields-with-select-type

 3. Add multi Select in  woocommerce checkout page 

   add_filter( 'woocommerce_form_field_multiselect', 'custom_multiselect_handler', 10, 4 );
function custom_multiselect_handler( $field, $key, $args, $value ) {

    $options = '';

    if ( ! empty( $args['options'] ) ) {
        foreach ( $args['options'] as $option_key => $option_text ) {
            $options .= '<option value="' . $option_key . '" '. selected( $value, $option_key, false ) . '>' . $option_text .'</option>';
        }

        $field = '<p class="form-row ' . implode( ' ', $args['class'] ) .'" id="' . $key . '_field">
            <label for="' . $key . '" class="' . implode( ' ', $args['label_class'] ) .'">' . $args['label']. $required . '</label>
            <select name="' . $key . '" id="' . $key . '" class="select" multiple="multiple">
                ' . $options . '
            </select>
        </p>' . $after;
    }

    return $field;
} 

add_action('woocommerce_after_order_notes', 'my_custom_checkout_field');

function my_custom_checkout_field( $checkout ) {

    echo '<div id="my_custom_checkout_field"><h3>'.__('My Field').'</h3>';

    woocommerce_form_field( 'my_field_name', array(
        'type'          => 'multiselect',
        'class'         => array('my-field-class form-row-wide'),
        'label'         => __('Fill in this field'),
        'placeholder'   => __('Enter something'),
        'options'       => array(
            'Buick' => __('Buick', 'woocommerce' ),
            'Ford' => __('Ford', 'woocommerce' )
        )
        ), $checkout->get_value( 'my_field_name' ));

    echo '</div>';

}

More Info Here 

http://stackoverflow.com/questions/13684533/woocommerce-custom-fields-multiselect