Hello!
So, the error message you're getting is because a colon (:) in a URL generally indicates a port. There was a time when HTTP websites authenticated using user:pass@, but that time has long since passed, and RTMP doesn't use that formatting to my knowledge.
In order to authenticate RTMP through a URL, you'll probably have to add the username and password to the end of the URL, as query parameters. That means you'll have a question mark (?) followed by the username and password in some configuration. Exactly what configuration that is, I can't say unless I know what software you're using for the actual RTMP server (or what website you're re-streaming to that requires authentication.) For Nimble Streamer, the end of the URL is "?rtmpauth=push_login:push_password" (the colon doesn't mess anything up here because the question mark means it's a query parameter, so it can't be a port.) For Wowza, the end of the URL might be something like "?user1&pass1", but you might need to set some extra configuration files up.
What website/software are you streaming to?