Just bought a RB750G - been using pfsense for years.

I would like NAT to work only from specific ip-adresses.

For an example: from my work i do remote desktop to my home computer.
I want NAT from wan port 1234 to internal pc port 3389
but only from my work ip-address, don't want to expose to the whole internet.
RB750G is placed at my home.

Can't figure it out

well u can create a dst-nat rule on ur router...i'm assuming that u have a public ip address

this is an example let's say ur ip addres of your office is 50.50.50.5 and your home router is 60.60.60.6 and your pc which is on your home LAN is 192.168.1.5

you would have to create this nat rule

/ip firewall nat add chain=dstnat src-address=50.50.50.5 dst-port=1234 protocol=tcp action=dst-nat to-addresses=192.168.1.5 to-ports=3389

hope that's what u wanted to know...