You have to first create a Pass List on the PASS LIST tab.
Then on the INTERFACE SETTINGS tab for the interface you want to set a custom HOME_NET for, choose the Pass List you created in the drop-down (the one that probably says “default” right now).
Navigate to Firewall → Aliases.
TIP: Remember that on pfSense you can nest aliases (so you can put an alias in another alias).
Navigate to Services → Suricata → Pass Lists.
Navigate to Services → Suricata → Interfaces.
In Networks Suricata Should Inspect and Protect: