Hello! I want to relay the live video signal through the live broadcast option CUSTOM RTMP but there is no information on how to use it. Someone tried this option?

Yeah, works great

What platform are you looking to stream through? I personally am not familiar with the custom RTMP option, but I'd like to see what I can find to help.

Hopefully I am understanding your post correctly, so here goes - There needs to be a platform to live stream to, process, and then forward live stream to those who are connected to the live stream session.
Screenshot from QNAP to showcase the RTMP platform stack-

I believe, there are free sites that allow RTMP connections to their platforms but I haven't tried it myself.

Be simple ) Youtube have RTMP protocol.

I stream a CCTV camera of my garden (Cat Cam) to youtube from a RPI3 using RTMP. It's pretty straight forward. The URL is made up of their base plus the provided ID. They used to have a guide but now they are pushing "verified encoders" more, it's hard to find. e.g. I use "rtmp://a.rtmp.youtube.com/live2/XXX-YYY-ZZZ"