Some notes on using that:
First, read all of the text descriptions on the new fields that show up when you switch to TAP. The notes are important.
Add a new VPN instance, select tap, fill in all your other info as you want, If you want to provide DHCP to clients, check the box to bridge DHCP, select the interface you will be bridging to, and (optionally) fill in the DHCP server pool. If you fill in the DHCP server start/end it should be a range of IPs outside of your existing DHCP pool. If you leave the IPs blank, it will pass DHCP through to your LAN DHCP server.
After you save the VPN settings, go to Interfaces > (assign), assign the new VPN interface. Go to Interfaces > OPTx, enable, leave IP type as "none", save. Go to Interfaces > (assign), bridges tab, bridge the VPN interface and your LAN or whatever internal interface(s) you want.
Go to Firewall > Rules, on the VPN interface be sure to add rules there that will pass DHCP and whatever other traffic you want (or just pass any/all).