Using ngrok to share your RTMP server is simple using TCP endpoints. 
ngrok tcp 1935
Once the ngrok agent starts, you can use the TCP endpoint in your configuration where you would normally see the IP address. For example: 
rtmp://your-ngrok-tcp-address:ngrok-port/live/<some stream name>
Streaming data can eat up a lot of bandwidth quickly. If you are on a free plan, you may run into the data transfer limits and need to upgrade to a paid plan.