You need to do two things to set up what you want. You need to set up the port forwarding and then also allow the traffic through the firewall.
In ASDM, go to Configuration -> Firewall.
To allow traffic through the firewall, go to Access Rules and click Add, then enter these values:
Destination: 188.8.131.52 (your external IP address)
To add the port forwarding rule, go to NAT Rules (below Access Rules) and click Add, then Add Static NAT Rule..., then put these in:
Use IP Address: 184.108.40.206
Port Address Translation (PAT)
Enable Port Address Translation (PAT)
Original Port: 3389
Translated Port: 3389