Add a checkbox in checkout page for customer whether wants to be a member & send customer details to admin email

functions.php

/**
 * ADD BECOME MEMBER CHECKBOX AT CHECKOUT PAGE FOR CUSTOMER
 */
function tkd_add_checkout_checkbox() {

    woocommerce_form_field( 'checkout-checkbox', array( // CSS ID
        'type'          => 'checkbox',
        'class'         => array('form-row mycheckbox'), // CSS Class
        'label_class'   => array('woocommerce-form__label woocommerce-form__label-for-checkbox checkbox'),
        'input_class'   => array('woocommerce-form__input woocommerce-form__input-checkbox input-checkbox'),
//        'required'      => true, // Mandatory or Optional
        'label'         => 'Want to be a member?', // Label and Link
    ));
}

add_action( 'woocommerce_review_order_before_submit', 'tkd_add_checkout_checkbox', 10 );

/**
 * Alert if checkbox not checked
 */
function tkd_add_checkout_checkbox_warning() {
    // its just a check that this function can get the POST value and do the necessary.
    if ( (int) isset( $_POST['checkout-checkbox'] ) ) {
        $to = 'yourmail@here';
        $subject = 'Mail Subject Here';
        $headers = array('Content-Type: text/html; charset=UTF-8');
        $msg = '';
        $data = [];
        foreach($_POST as $key => $value) {
            $data[$key] = $value;
            $msg .= $key . ' : ' . $value . '<br>';
        }
        $mail_body = 'Name:'.$data['billing_first_name'].' '.$data['billing_last_name'].'<br>Address:'.$data['billing_address_1'].' '.$data['billing_address_2'].' '.$data['billing_city'].' '.$data['billing_postcode'].'<br>Phone:'.$data['billing_phone'].' <br>Email: '.$data['billing_email'].'<br>';
        // We can use wp_mail with POST value for sending mail instead of update_option function
        wp_mail($to, $subject, $mail_body, $headers);
    }
}

add_action( 'woocommerce_checkout_process', 'tkd_add_checkout_checkbox_warning' );