Step 1: Establishment of a PPP link between two sites
This can be done very easy. For example you may use PPTP or SSTP to link up two sites. In general the server should be located in HK and the client should be located in the Mainland China.
Step 2: IP-based policy route using the packet-marking method
1) Set the IP address of a PC to be connected to the HK VPN server, e.g. 192.168.99.101
2) Using the "Mangle" function to mark the packet.
/ip firewall mangle add chain=prerouting src-address=192.168.99.103 action=mark-routing passthrough=no