subscription-button.php

add_action('woocommerce_after_add_to_cart_button', function(){
    global $product;
    
    if ( ! $product ) {
        return;
    }
    
    $product_id = $product->get_id();
    if(!$product->is_type('subscription')){
		return;
	}
    echo '<input type="hidden" name="add-to-cart" value="' . esc_attr($product_id) . '">';
});