Depends on what you mean by "accessing" your router. If you want to connect with https, then yes, you should make a dst-nat that has a dst-port 8090 and forwards to 192.168.99.1:443.
The "I want more info" version: The idea is to make a computer in the NATed network available from the outside. So if you know your external IP (or DNS name), you can just access that IP on 1 port that you don't use on the external router, like 8090, and just forward it to your internal router to the port you want/need.
Mikrotik wiki artice about dst-nat