// Multiple products, multiple quantities per product.
$product_ids = '63833:2,221916:4';
$add_to_cart_url = esc_url_raw( add_query_arg( 'add-to-cart', $product_ids, wc_get_checkout_url() ) );

// Multiple products (default quantity of 1)
$product_ids = '63833,221916';
$add_to_cart_url = esc_url_raw( add_query_arg( 'add-to-cart', $product_ids, wc_get_checkout_url() ) );

// Normal add-to-cart URL
$product_ids = '63833';
$add_to_cart_url = esc_url_raw( add_query_arg( 'add-to-cart', $product_ids, wc_get_checkout_url() ) );