I have a windows server 2012R2 hyper-v with pfsense, among other things.
I suggest you use the following setup:
Create two virtual switches. Call one WAN and the other LAN.
For the WAN switch, use external network, since it's an external NIC. Do not select allow the management operating system to share this nic.
For the LAN switch, use external network, since it's also an external NIC. However, do select allow the management operating system to share this nic, so you can access the hyper-v from within the LAN.
When you create the guest for pfsense, create it with two network adapters, one connected to each of the switches. When you install pfsense, you will need to know the MAC addresses of the two adapters so you can select hn0 and hn1, accordingly.