/ip firewall filter
add chain=input connection-state=established action=accept
add chain=input connection-state=related action=accept
add chain=input action=drop protocol=tcp in-interface=ether1 dst-port=8090
add chain=output action=accept
Of course adjust that to also account for the result of your rules. the lines with 'connection-state' should be the first listed in the input chain.
The idea is that you want to ensure that while traffic initiated to the router should be dropped, but that all connections that the router itself established should be unconditionally allowed.
I do not use the built in (or any other) proxy so this is a guess, but at worst it just won't have any effect.