I have a test application running at test.rsscloud.io on ports 80 and 9876.
The following code works:
curl --location --request POST 'https://brokenriverbooks.com/?rsscloud=notify' \
--header 'Content-Type: application/x-www-form-urlencoded' \
--data-urlencode 'domain=test.rsscloud.io' \
--data-urlencode 'port=80' \
--data-urlencode 'path=/feedupdated-s8759' \
--data-urlencode 'registerProcedure=' \
--data-urlencode 'protocol=http-post' \
--data-urlencode 'url1=https://brokenriverbooks.com/feed/'
However if you change the port to 9876 it fails. For some reason the rssCloud plugin seems unable to hit ports that are not 80.
Here are items from my debug session (https://andysylvester.com/2022/12/04/rss-cloud-support-in-wordpress-com-not-working/):
===
Made a post (https://rsscloud4.wordpress.com/2022/12/04/test-post-007/) at 2:18 pm, no response to my test script.
Checked in FeedLand for new item, did not see it until 2:28pm (10 minutes later). In testing of other tools creating RSS feeds with rssCloud support, the item would appear in FeedLand within a few seconds. I have noticed that feed readers seem to take up to 30 minutes to detect that there has been a change in a RSS feed from WordPress.com or WordPress.org sites.