Keep in mind this feature is initial implementation and a part of more complex BGP debugging tools currently work in progress.
At the time of writing this post advertisements can be seen by dumping outgoing advertisements to pcap file.
You will need to:
*) enable “output.keep-sent-attributes” in BGP connection settings
*) run "dump-saved-advertisements " with specified session id and filename specified with “save-to” from the BGP session menu
*) output then can be seen by limited pcap viewer in “/routing/stats/pcap” menu or by downloading pcap file and viewed in Wireshark.
Before trying to use this feature make sure that that there is enough free RAM and free space on HDD. 800k routes will take around 120MB RAM and more than 300MB of HDD if dumped to file.