Unfortunately the Nginx package for FreeBSD does not contain the RTMP module, so you need to compile it either from upstream sources or use the FreeBSD ports and enable the RTMP module before compiling:

# portsnap auto
# cd /usr/ports/www/nginx
# make config
# make
# make install

now create a new file name rtmp.conf in /usr/local/etc/nginx using your favorite editor, i use neovim:

# cd /usr/local/etc/nginx
# nvim rtmp.conf

now paste following code to rtmp.conf:

rtmp {
    server {
        listen 1935;
        chunk_size 4096;
        application hlslive {
            live on;
            hls on;
            hls_path /usr/local/www/nginx-dist/hlslive;
            hls_fragment 3s;
            hls_playlist_length 18s;
        }
    }
}

save it and open nginx.conf and add the following:

load_module /usr/local/libexec/nginx/ngx_rtmp_module.so;  # <-- must be loaded at the top of the file
include rtmp.conf;  # <-- can be at the end of the file

check if the config is valid:

# nginx -t

if valid you can start your Nginx/RTMP server and send the first RTMP to it.