Add this code to the functions.php file of your theme, at the end of the file. You can also find the file in Appearance > Editor > Theme Functions (functions.php)

add_filter( 'woocommerce_billing_fields', 'bp_add_preorder_date_field_in_checkout',100 );

if( !function_exists( 'bp_add_preorder_date_field_in_checkout' ) ){
    function bp_add_preorder_date_field_in_checkout( $fields ){
        if ( 'no' === get_option( 'wc_preorders_always_choose_date' ) ) {
            $class = ['disabled-input', 'form-row-wide'];
        } else {
            $class = ['form-row-wide'];
        }
       
        $fields['preorder_date'] = [
            'label'             => __( 'Pre order Date', 'preorders-for-woocommerce-pro' ),
            'type'              => 'text',
            'class'             => $class,
            'description'       => __( 'Please enter the date when you want to receive your order', 'preorders-for-woocommerce-pro' ),
            // 'input_class'   => 'datepicker',
            'priority'          => 35,
            'required'          => true,
            'default'           => apply_filters( 'preorder_oldest_date', null ),
            'custom_attributes' => ['data-pre_order_date' => apply_filters( 'preorder_oldest_date', '' )],
        ];
        return $fields;
    }
}

Pre-orders for Woocommerce