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.
I've uncovered a couple of things regarding this.
The rsscloud plugin doesn't specify a scheme for the URLs so although it seems to work in some situations on wordpress.com on an up-to-date (6.1.1) self-hosted blog it always fails. "A valid URL was not provided"
wp_http_validate_url seems to only allow URLs with ports 80, 443, or 8080 so aggregators running on other random ports fail.