This post contains a working example of a port forwarding configuration on a
Cisco ASA 5505 that’s allowing RDP, TCP port 3389, through the firewall to from
the Internet to the LAN side to a server.

Port translation is used here. Where remote user RDP destination port TCP 4690/4691
instead of using the default port 3389. On ASA we have a port translation rule where
4690/4691 tcp ports are translated to 3389 and forwarded over to internal Lan RDP server.

Please see Configuration as Below :

object network KBhayani_rdp
host 10.50.50.7
!
object network icaldwallader_rdp
host 10.50.50.8
!
object-group network RDP_PCs
description Remote RDP access
network-object object KBhayani_rdp
network-object object icaldwallader_rdp
!
object-group service RDP_PORT_IN tcp
description Remote RDP access
port-object eq 3389
!
!
object network KBhayani_rdp
nat (inside,outside) static 85.234.75.213 service tcp 3389 4690
!
object network icaldwallader_rdp
nat (inside,outside) static 85.234.75.213 service tcp 3389 4691
!
!
access-list outside_in extended permit tcp any object-group RDP_PCs object-group RDP_PORT_IN