In virtual switch manager , during creating external virtual switch you need to select an physical NIC from the drop down list under "external network" .
If you checked "Allow management operating system to share .." there will be an virtual NIC with the name like " vEthernet(virtual switch) " in network connections of hyper-v host (this virtual NIC will inherit the properties of original physical NIC , MAC and IP Address ,etc..) .
After creating the external Vswitch , you may connect the VM's NIC to that virtual switch in VM settings .