NetBIOS name resolution is a layer-2 broadcast based name discovery protocol. Layer-2 broadcasts do not traverse layer-3 boundaries such as the Client VPN interface on an MX.
WINS is service that provides centralized name resolution of NetBIOS hostnames. NetBIOS clients register their hostnames on the WINS server and other NetBIOS clients query the WINS server to resolve NetBIOS names.
To allow hosts that utilize NetBIOS names to find network resources over Client VPN, specify the IP address of a WINS server in the Client VPN configuration. This is done using the WINS setting on the Configure > Client VPN page.
In the screenshot below the specified WINS server is: 192.168.1.100