VBoxManage modifyvm "fill in VM name" --natpf1 "guestrdp,tcp,,53389,,3389"
natpf1=nat Port Forwarding rule1. You can also use natpf2, natpf3 etc.
guestrdp=just a description
53389= port to connect to from outside. Connect to host computer port 53389 will be forwarded to guest port 3389